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.
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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.
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.