Genz
On the game's final level, New Hiessgart Castle, you must win four separate boss battles. The first features Colonel Genz Bresslau. Genz shoots fireballs that follow you, so be quick to avoid them and try to keep moving. To have any chance, make certain to heal Alphonse to full strength and enter the battle with plenty of frenzy potions in your inventory. Once you exhaust all frenzy attacks, transmute the statues near the door into cannons and fire away until you run out of ammo. Keep reloading and attacking until Genz falls.
Three Chimeras
The next boss fight requires you to defeat three chimera monsters: a behemoth, a bird and a sea serpent. Transmute a cannon near where the sea serpent appears and blast away. Once you've stunned the serpent, go behind it and hack away with your sword. Reload the cannon and attack the behemoth next, forcing it to eject its shells. Transmute the shells and use them to block the behemoth's vacuum attack. Now blast it with four bombs. Then retreat to the cannon to gun down the bird chimera. Remember to always keep moving, and you may want to use an agility cap to help Edward avoid the chimeras' fireball attacks more easily.
Camilla
Camilla, a mysterious bounty hunter, goes into battle alongside the Ultimate Chimera. Try to take out the chimera first, transmuting whatever you can find into a cannon and shooting it dead. Next, take out Camilla, who has a particular weakness for crossbow attacks. Wait for Camilla to land after unleashing her vortex assault and then drill her with the crossbow. One shot can be all it takes.
Armstrong and Mustang
The game's final boss battle pits you against Armstrong and Mustang, two powerful alchemists. Before the fight, give Edward a magic cap to raise all your stats and then distribute your collected bonus points between Alchemy and Attack. Focus on Armstrong first, because he's easier to hit. Use the Wind Dagger and combo Armstrong from behind while he does his special rock attack. Next, use the tanks on either side of the screen to blast Armstrong, remembering to alternate tanks to prevent them from taking damage. Keep repeating the same sequence until you're left with just Mustang. A single tank blast knocks Mustang into the air, allowing you to get off a 5-hit combo. Also, attack with a melee combo just before Mustang performs his meteor attack.