At the main menu, press Up(2), Right, Left, Down(2), Start. While playing the game, go to an Xtraction save location and use the button next to "Skin".
Successfully complete the game to unlock the classic costumes (Bobby Drake/original Iceman, 1990's Colossus/without metal, original Beast, 1980's Cyclops, original Jean Grey/Phoenix, original Nightcrawler, street clothes Magma, 1990's Wolverine/Weapon X).
Successfully complete the Nuclear Facility level to unlock Colossus.
Defeat Mystique on the New York City Riot level to unlock Cyclops.
Locate Emma Frost in X Mansion after the Nuclear Facility level to unlock her.
Successfully complete Magma's second training round to unlock him.
Enter the Astral Plane for the first time to unlock Professor X.
Successfully complete the New York City Riot level to unlock Psylocke.
Successfully complete the first level to unlock Ice Man, Jean Grey, Beast, Rogue, and Storm.
Successfully complete the second level to unlock Jubilee and Nightcrawler.
Successfully complete the third level to unlock Gambit.
Collect all 19 Sketchbooks to unlock the artwork in Colossus' room.
Collect the hidden Comic Books to increase your characters' stats.
Collect the hidden Danger Room Discs to unlock more tasks in the Danger Room.
When you receive a new course in the Danger Room, do not complete it. Instead, fail as much as possible. Every time you do the course, you will receive health packs, energy packs, armor, etc. Additionally, the best way to get a extra utens is to fail the Danger Room courses. If you complete a course, the next time you send in your team they will not receive anything for their trouble. However, the first "final exam" as a freshman is a very easy one. Just do not destroy the generators. By doing this, your team can run around kicking butt for quite awhile and collect a lot of times. Note: Make sure all your teammates are set to auto-refill their health at 80% (this can be changed in the first field of the detail view for your characters). Also, watch for the inevitable cut off time (about a half hour in). At this point, the villains will no longer give anything up. Then, destroy the reactors and leave. Make sure to sell Forge anything you absolutely do not need; otherwise about halfway through your available time your inventory will be full and you can miss out on some good items.
To do this trick, first have Storm and Cyclops learn the Leadership ability. This works best if the skill is at the maximum. Once done, always have Cyclops and Storm in your party with two other members that you want to level up. Start fighting your enemies, except this time use your abilities more often. This ability will work once you execute a combo with either Storm or Cyclops' abilities. It should give you extra damage and experience points. To make this even more beneficial, try your best to execute a combo with both Storm and Cyclops' abilities. This should double the normal amount of bonus experience and damage. The Leadership ability's maximum capabilities is at 150% damage bonus, and 25% experience bonus; and if you managed to combo both Storm and Cyclops' abilities this should dish out a 250% damage bonus and 50% extra experience. This trick should be used in the Danger Room Focus 601 or Qualifying Exam 400. These are the areas that give most experience points with the most time for you to dish out your attacks.
Make sure you find the "Focus 601" Danger Room disc. Once you have passed the exams (Qualifying 100, 200, 300, 400, and 500) and can access the "Legends" menu, select "Focus 601". In this mission, you must defeat five Master Acolytes, which will give you over 297,000 experience points each. Each time that you do this mission successfully, you are guaranteed at least 1,100,000 experience points.
Go to qualifying exam 400 and keep failing it. Do not worry about Professor X; just keep killing Astral Fury about for 117,000 experience each time. Colossus can kill them in one hit.
This trick works best at the start of the game. Have Rogue in your party and level her up quite a bit. Now once you have skill points, use it on Southern Strike/Smash and increase the abilities level as far as possible while you are playing the game. Use it on your enemies and it should either kill or take out a large amount of HP. Combine with Leadership to deal more damage.
Have Wolverine and Beast in your party. Choose Beast and face Wolverine. Press Throw and Beast will pick up Wolverine and hurl him at an enemy, doing a large amount of damage. Note: Rogue can also throw Wolverine just like Beast.
This is the signature move of Wolverine and Colossus combined (but can also be done with Beast). Play as either Colossus or Beast and press X to throw Wolverine.
Start the combo by pressing B then A(2), B(2), then keep repeating A(2), B(2), A(2), B(2), etc. It is a lot easier if done slowly and you wait for the character to swing.
To defeat Mastermold, first use Wolverine and slash away at his feet and jump when he steps. When the three Sentinel Controllers come out, fight them and dodge Mastermold's missles by running in circles around the generators. When he gets his shield, turn on the three generators and wait until it goes away. From now on, use a long range fighter to finish him off. Remember to dodge the missiles.
Choose Wolverine (at a high level) and run behind the Sentinel. Choose the Brutal Slash attack (Black + A) and repeat it until the Sentinel falls.