Adamantium is the highest possible grade you can earn in a match. The amount of XP needed is 4,000. You earn XP by doing basic moves, super moves, and you also earn xp based on how well you defend yourself. An easy way to earn this is to start a 2 player versus match and play by yourself. When the match starts, just string combo and tag combo super moves together and by the end of the match you should have earned enough for the achievement.