This is a trophy that would be impossible without the proper settings. For starters, lower the handicap on a second controller so it has less health and raise your own (I'm unsure if this has any bearing on attack, but it doesn't hurt). After this, pick a team that has Magneto followed by any other 2 characters for you. The other team should be comprised of 3 "Roll"s. As soon as the match starts, move up toward her and press "Down and Triangle". It should perform a move that launches her in the air and takes off a decent chunk of health. Keep spamming this as rapidly as you can, quickly knocking out each opponent. If you've done this within 15 seconds, you have your trophy!