The fourth level of the game is Apocalypse's Fortress. The first stage is The Climb, where you face a boss named Tusk. The second is inside the fortress, where you face multiple Tusks. The third is the Computer Room, where you will face off with Apocalypse.
The Climb[edit]
Tusk[edit]
Tusk stands in the center of a raised platform and never gets off of it. His only attacks are throwing rocks at you, summoning mini-Tusks to attack you, and a counter-attack that occurs once you've dealt damage to him. If you hit him on his right side, he will charge to the right in an attempt to hit you. If you attack him on his left, he charges to the left. If you jump onto his platform he will not throw rocks or summon mini-tusks, but will charge at you. Best strategy is to jump onto the platform, strike Tusk, jump over him as he charges at you onto the other side of the platform and repeat. Confident players would use Beast as he will destroy Tusk the quickest, although first timers may want to use Wolverine given his healing factor.
Another option, if you are having extreme difficulty with defeating him, is to use Psylocke and upon reaching him remain directly center underneath the pillar and strike up continuously with your sword. If you keep swinging the sword without stopping you will prevent the minis from even getting off the pillar, cut the rocks that Tusk throws down at you, and hit him at the appropriate times killing him easily without taking any hits whatsoever. It may take roughly 8-10 hits to kill him using this method.
Inside[edit]
Multiple Tusks[edit]
See the Tusk strategy above.
Computer Room[edit]
Apocalypse[edit]
In this fight, the majority of the stage is jumping and ducking over explosive orbs. Orb bombs come down from the air and circle around the main computer. To end the orb rotations faster, you must hit the red and blue orb that comes down. Occasionally Apocalypse comes down on a floating platform. His only attack is using his extendable arm to attack you diagonally. As you deal more damage to him, he becomes faster and more aggressive. Wolverine is the optimal character here as it is not hard to maintain peak health and striking Apocalypse with his peak mutant slash will make Apocalypse flee before the computer is even destroyed. Magneto can be made to hover above the bombs out of harms way, however it can be difficult to shoot the blue orb, his regular attack is very weak so the battle is needlessly prolonged and occasionally Apocalypse can strike at an angle that clips the hovering Magneto. Psylocke would be a better bet (her high jump comes in handy) or else Cyclops.