This Boss is a lot trickier than Guard Scorpion, as every attack causes a fair amount of damage and AB counter-attacks your party when they attack!
An easy way to beat him is to get each character's Limit Bar full and unleash the attack when AB's back is turned towards that character. As the party has to split up for this battle, (Cloud on the right,
Barret and Tifa on the left) use Cloud's Limit when AB is facing Barret and Tifa and Barret and/or Tifa's Limit when AB is facing Cloud.
Doing this on my game I beat AB with only two Limit Breaks!
Sadly it might not work if your Limits are not strong enough and/or you strike when AB is facing the attackihg character, causing less damage.
So try attacking when AB is facing the other way and as a precaution you can have Tifa on standby to cure, (either Potions or Cure + All works fine) when your party is injured.
AB's Big Bomber attack can be pretty deadly, so keep an eye on HP levels. As this Boss is a machine, Cloud's 'Bolt' Materia works very well.
After the battle you'll be rewarded with a Titan Bangle, congratulations!

 APS is a fairly easy Boss, he'll use a Tsunami attack which causes the Sewage water to wash over you, (yuck!) it deals a lot of damage but fortunately harms himself as well!
On my game, he used the attack and killed himself, the irony! ^_^
Anyway, use the 'Fire' and 'Lightning' Materia on him to cause some damage. As soon as anyone has a Limit, use it! Aeris' 'Healing Wind' should also come in useful as you'll
need to heal often.
If Aeris and Tifa have low Maximum HP, then you can equip Cloud with 'Cover' Materia to take most of the damage, which also increases his Limit so you can use it more frequently.
After the battle you'll get a Phoenix Down, not a fantastic prize, but useful nonetheless.

 Reno is pretty nasty, he uses an electric rod attack that deals a fair amount of damage, he also uses 'Pyramid' which traps a party member so that he/she can't attack!
The only cure is to attack that party member directly to free him/her, this, strangely enough, doesn't cause any damage.
Use 'Fire' and 'Ice' Materia against him, but do not use 'Lighting' - it doesn't do much damage as it is Reno's strength!
Unleash as many Limits as you can on him, curing/freeing party members occasionally and the battle will soon be over.
You'll win an Ether when you've defeated him, which will be useful for your next mission...

 H0512 uses Poison based attacks, so if you have an 'Elemental' Materia, you can link it to that 'Poison' Materia, (found in a chest next to the last Save Point) onto
someone's Armor to protect from Poison, you should also have at least one Star Pendant Accessory by now to protect another character. Apart from that, there's nothing
you can do about the Poison, so I hope you have some Antidotes or Remedies!
H0512 is joined by three wimpy Sample: H0512-OPT, (see below) which attack your party. They don't cause much damage, so just leave them alone and concentrate on
attacking H0512 head on. (H0512-OPT regenerate so there's no point anyway).
As for H0512, use Limit Breaks, physical attacks, 'Fire' and 'Lightning' Magic, but don't use 'Ice'! You'll also be fighting with a new party member; Red XIII, who has 'Fire' + 'All' and 'Sense'
Materia equipped which comes in useful.
Remember to 'Cure' often and that 'Poison' causes continuous damage, so you'll want to cure it as soon as possible!
For all your hard work you'll get a Talisman Accessory, well done... But it's not over yet!

 Just leave these H0512-OPT alone and focus on H0512, their attacks are wimpy and they regenerate when you kill them anyway.

 They just keep getting harder don't they?
Hundred Gunner is a tough one, it attacks you just as you're going down in the lift, however with it being in the lift opposite you, you can't use physical attacks.
Use Barret as your attacker, (he has a Long Range weapon) and equip Red and Aeris with 'Lightning' Materia, you should have two if you bought another from a Materia Shop.
Then give a party member a Restore + All linked together as your healer, it would be best to give Barret, (or a party member without a 'Lightning' Materia equipped) the healing job.
Hundred Gunner will be using some nasty attacks, but definitely watch out for it's 'Wave Artillery' as it causes damage to all party members!
Whenever one of your party members has a Limit, use it quickly. Red and Barret have good attacking Limits that deal a fair amount of damage. Aeris' Limit is the most valuable for this Boss though, as it heals all your party members for a generous amount.
So, attack with Barret, use 'Bolt' as much as possible, heal frequently with a 'Restore' + 'All' or Aeris' Limit, and whenever Barret or Red have Limits, use them!
Oh and BTW, it's not over yet...

 What?! Where'd this thing come from?!
Yep, straight after Hundred Gunner you have to fight a, (fortunately not quite as tough) Heli Gunner... Great.
Anyway pretty much the same tactics here as Hundred Gunner, attack with Barret, use 'Bolt' as much as possible, heal frequently with a 'Restore' + 'All' or Aeris' Limit, and whenever Barret or Red have Limits, use them!
Heli will have some different attacks, and will fly, but it's nothing to worry about. If you missed the strategy for the last Boss, look below for more tips...

Use Barret as your attacker, (he has a Long Range weapon) and equip Red and Aeris with 'Lightning' Materia, you should have two if you bought another from a Materia Shop.
Then give a party member a Restore + All linked together as your healer, it would be best to give Barret, (or a party member without a 'Lightning' Materia equipped) the healing job.
Heli will be using some nasty attacks, but definitely watch out for it's Cannon attacks as it causes a good bit of damage!
Whenever one of your party members has a Limit, use it quickly. Red and Barret have good attacking Limits that deal a fair amount of damage. Aeris' Limit is the most valuable for this Boss though, as it heals all your party members for a generous amount.
So, attack with Barret, use 'Bolt' as much as possible, heal frequently with a 'Restore' + 'All' or Aeris' Limit, and whenever Barret or Red have Limits, use them!
Oh and BTW, it's still not over yet...

 Ok, now to get battling Rufus, and BTW Cloud is gonna be on his own this time, but you get to sort out your Materia before the battle, fortunately.
Equip Cloud with a 'Restore', (on it's own, no All's needed this time) and a 'Lightning', (with an 'All' this time) along with any other useful Materia you might have.
Rufus will be using a Shotgun attack which only does about 50 damage, but he'll be joined by a 'Dark Nation' which not only attacks you, but casts Barrier and MBarrier on itself and Rufus!
So first eliminate the Dark Nation using 'Lightning' Materia, (if you linked it to an 'All' it will also damage Rufus) attack, and Limit Breaks. Keep in mind you won't be doing as much damage due to the Barriers.
When the Dark Nation is down, focus on Rufus, his Barriers will soon wear off, allowing you to attack full power. Limits work brilliantly, so unless your HP is low, use them straight away!
You may have to 'Cure' every other turn though; Rufus will be attacking very frequently so watch your HP!
For all your hard work, you'll get a Protect Vest Accessory!

 Rufus will be joined by a 'Dark Nation' which not only attacks you, but casts Barrier and MBarrier on itself and Rufus!
So first eliminate the Dark Nation using 'Lightning' Materia, (if you linked it to an 'All' it will also damage Rufus) attack, and Limit Breaks. Keep in mind you won't be doing as much damage due to the Barriers.
For all your hard work, you'll get a Guard Source and extra EXP!

 Well, Shinra put up a good fight don't they? Don't worry this is the last one.
Anyway, Motor Ball will be using Fire based attacks, so if you have any way of protecting against Fire, equip it! As Motor Ball is a machine, using Lightning, (yet again) is a must.
I recommend Cloud, Red and Aeris for this battle, equip Red and/or Cloud with 'Lightning' Materia, and Aeris with 'Restore' + 'All'. Have Aeris in the back row, healing when needed, Red and Cloud in the front row, dealing physical attacks and casting Bolt.
Eventually Motor Ball will use 'Rolling Fire' this is bad news, it deals around 200 damage to all party members! Take advantage of this, by using any Limit Breaks that have filled up by MB and bringing it down! Of course you'll probably want to heal before this, so have Aeris use that 'Cure' + 'All' or her 'Healing Wind' Limit if she has it.
Before you now it, Motor Ball will be long gone and you're free from Midgar at last! You'll also be rewarded with the Accessory: Star Pendant. ^_^

 Yeah, it's really called Bottomswell. ^_^
Anyway, the 'Chocomog' Summon Materia I hope you got from the Chocobo Farm, works very well against Bottomswell, so make sure you use it against it.
Bottomswell will use an attack which traps a party member in a blue bubble, which slowly drains that character's HP! The only way to free them is to cast magic on the bubble. Don't worry; it won't damage the imprisoned character!
It'll also use a 'Big Wave' attack, damaging your party and itself. Which, (like APS) could be it's downfall! ^_^
Also, since Bottomswell is flying, physical attacks won't work. So use Barret's attacks and/or Yuffie, if you have her. You could also use that 'Long Range' Materia you should have from the Mythril Mines.
So, use 'Chocomog' Materia, 'Lightning' Materia (for attacking Bottomswell and freeing trapped party members), Limits (Aeris' Limit is useful as always), Curing when you need to and attacking with Long range characters. And after all that you'll get the Power Wrist Accessory, phew!

 Ok, this is a mean one. JENOVA will be using Laser attacks and may even attack twice in a row! The main thing to worry about though, is her 'Stop' magic. It will stop a party member's Time Gauge, meaning they won't be ably to do anything! I think a Remedy would fix the status, but if it doesn't, it may wear off instead.
Right, equip 'Chocomog' and your new 'Shiva' Summons to deal some damage, Enemy Skills work well, also having 'Lightning' is good for a party member with a weak attack.
Have Aeris in your party as your healer, occasionally casting spells. Cloud and another strong character, (Red or Yuffie will do) should be attacking full force, using as many Limits as possible.
Despite the HP, Jenova will soon be defeated and you'll get a White Cape Accessory!

 You'll be fighting Dyne with just Barret, so you might want to equip him with some good Materia first. Summons, 'Fire' Magic, 'Restore' (without 'All'), Enemy Skills and Limits work well.
You may need to cure Barret very frequently as Dyne may be taking almost half of your HP every turn! The only good thing about this, is that you can blast him with some Limits! ^_^
Molotov Cocktail is a nasty attack, so heal if your HP is below half way, before attacking.
For beating Dyne you'll get a Silver Armlet Armor, not bad.

 This battle pretty easy, but Summons are definitely recommended. 'Ramuh', 'Shiva' and 'Ifrit' are a must. Reno's attacks can be very annoying; doing plenty of damage, so use 'Restore' + 'All' to heal your party. You could also use Aeris' 'Healing Wind' Limit as an emergency cure.
After casting your Summons, use strong magic, (preferably with an 'All') and Limits if you have them.
Soon the battle will be won and the Fairy Tale Weapon for Aeris will be yours.

 This battle pretty easy, but Summons are definitely recommended. 'Ramuh', 'Shiva' and 'Ifrit' are a must. Rude's attacks can be very annoying; doing plenty of damage, so use 'Restore' + 'All' to heal your party. You could also use Aeris' 'Healing Wind' Limit as an emergency cure.
After casting your Summons, use strong magic, (preferably with an 'All') and Limits if you have them. Make sure to beat Rude quickly to stop him from curing himself and Reno.
Soon the battle will be won and the X-Potion will be yours.

 This battle can be either hard or easy. You can battle it out with your Summons and 'Ice' Magic, or you can use 'Cure' and Phoenix Downs.
Another annoying part of this battle is that your party will be worn out from those really hard Spiders before. You might not even have any Phownix Downs or MP for this Boss!
Anway, Gi Nattak will be accompanied by Soul Fires, which he can order to 'Take Over' one of your characters, causing continuous Fire damage. This can be converted to HP however if that character has a 'Fire' + 'Elemental' equipped on his/her Armor, or a Fire Ring Accessory. Otherwise, ignore the Soul Fires.
Using 'Shiva', 'Ice', 'Cure' or a Phoenix Down, is the best way to get his HP down. As Gi Nattak is an 'Undead' Fire type monster, Ice, curative magic and Items work very well.
But to instantly kill him, you can use a Phoenix Down to do so, it doesn't work 100% of the time though...
Limits are good attacks, but, funnily enough, not as good as Cure. ^_^
When Gi Nattak is finally defeated, you'll get Aeris' Wizer Staff, phew!

 Gi Nattak will be accompanied by Soul Fires, which he can order to 'Take Over' one of your characters, causing continuous Fire damage. This can be converted to HP however if that character has a 'Fire' + 'Elemental' equipped on his/her Armor, or a Fire Ring Accessory. Otherwise, ignore the Soul Fires, they regenerate anyway.

 The Materia Keeper is one of the hardest Bosses of the first Disc! Being able to Cure itself and cause massive damage is not good.
To start off, use 'Cross Slash' or 'Chocomog' to temporarily stun MK, giving you a head start. ^_^
If you defeated the Optional Boss, Lost Number, at Nibelheim, then you should have the 'Odin' Summon. It is very effective so use it as soon as possible.
Limits and the Enemy Skill, 'Aqualung', work briiliantly. Bio can also be used against MK.
Towards the end of the battle, MK will use 'Trine', although it causes major damage, make sure you have an Enemy Skill equipped to learn it.
Remember to not use 'Fire' and 'Ifrit' as MK aborbs Fire damage. And don't forget to watch your HP! MK causes lots of damage so 'Cure2' with an 'All' is recommended!
After that tricky battle you'll get a Jem Ring Accessory. Congrats!

 Compared to the Materia Keeper, Palmer is a pushover. He uses one attack called 'Mako Gun' that causes Fire, Ice and Lightning damage, but it is not to be worried about.
Use Summons, Limits and physical attacks, also 'Bio' is pretty effective.
Soon his measly 600 HP will have vanished and he'll taunt you while running away...
... Splat. ^_^

 Right, as you probably know, 'Fire', 'Ifrit' or 'Kjata' won't work as RD is a Fire based monster. So what do you use? No, not butterflies... *Rolls eyes* 'Ice' or 'Shiva' work very well because they are the opposite... Element. Well done! See, you're getting the hang of it now! ^_^
Ahem, use Summons, Limits, Enemy Skills, 'Ice' Magic and don't forget to Cure!
Good luck! Oh and equip the Dragon Armlet Armor, it's pretty good!

 Demon Gate, pretty much the hardest Boss of Disc one... Not only does he have the highest amount of HP-MP, (10,000 HP!) he has the most devastating attacks.
'Rock Drop' where a large stone falls on top of a character, doing around 600-800 damage; 'Cave In' and 'Demon Rush' which both deal about 900-1200!!
Ok, strategy time! Equip your best armor, use the Enemy Skill 'Big Guard', and always have a healing character on standby to cure your party, preferably in the back row.
'Cure' + 'All' for your main curing method, but you can also use Aeris' Limits. If you have 'Great Gospel', (which you should have by now) use it as often as possible!
Now for damage, don't use Elements, they are not the way to win this battle, you might as well flush 'Shiva', 'Ifrit', 'Ramuh' and 'Kjata' down the toilet for all Demon Gate cares... ^_^
Limits, physical attacks, (these work surprisingly well) and the newly aquired 'Bahamut' is a must!
Enemy Skills can also be used, 'Trine' and 'Aqualung' work well.
After your hardest battle yet, you'll get a Gigas Armlet Armor, well done!

 Yes, Jenova is back and tougher than ever! Although she is relatively easy, (seeing as your gaming spirit is crushed by the previous scene) and you'll need to watch out for certain attacks.
Blue Light and Flame are moderate attacks dealing around 500 damage, but Jenova will using them 2-3 times a turn so keep those HP levels up! Casting magic is useless against her as she uses 'Reflect' to bounce the magic onto your party. So whatever you do, do not cast 'Reflect on yourself! It won't solve anything, it'll just make any magic bounce between the two sides eternally!
Instead use physical attacks, Limits and Summons. 'Titan', 'Bahamut' and 'Kjata' are good for eliminating some HP.
She will also use 'Aqualung' which is pretty devastating for your party, counter this by having 'Leviathan' + 'Elemental' linked on your Armor, or equipping that Water Ring Accessory you got earlier.
So give it all you've got, (despite the previous scene) and avenge your friend! You'll be awarded a Wizard Bracelet Armor for your hard work. Congratulations.