Here are some hints for beating Tusk with various characters:

Cyclops: Keep doing your laser move
Psylocke: When he is dashing, do your special attack
Gambit: Keep throwing cards
Wolverine: Hit him as much as you can
Beast: Keep head butting him

Get him to a edge. Push him out with your attack so that he will fall in the acid. He will get more damage this way.

Complete the game without ever using passwords to unlock a different ending.

Exodus-Gambit, Gambit, Magneto, Wolverine, Beast, Apocalypse, Xavir, Magneto

Tusk-Psylocke, Xavier, Wolverine, Psylocke, Cyclops, Xavier, Cyclops, Wolverine

Apocalypse-Gambit, Cyclops, Cyclops, Wolverine, Cyclops, Wolverine, Cyclops, Xavier

Omega Red-Xavier, Psylocke, Apocalypse, Psylocke, Apocalypse, Psylocke, Xavier, Apocalypse

Juggernaut-Psylocke, Xavier, Magneto, Wolverine, Wolverine, Apocalypse, Xavier, Gambit

Level 9-Gambit, Cyclops, Beast, Cyclops, Wolverine, Cyclops, Psylocke, Beast

Level 10-Beast, Magneto, Magneto, Wolverine, Apocalypse, Magneto, Xavier, Gambit

Level 11-Xavier, Psylocke, Apocalypse, Psylocke, Magneto, Gambit, Xavier, Magneto

Genoshan Forest-Beast, Magneto, Xavier, Wolverine, Apocalypse, Beast, Cyclops, Wolverine