A fairly easy trophy. To unlock a minion you must get any character to level eight this will be done by using the character (finishing a arcade mode will give more then enough experience) When that's done go to the main menu and go to customize and under that profile, when there find the minion you have unlocked and select it, which will equip. Now to show it off, you need to get a double or triple kill in any mode that is excluding practice mode.
When you are playing (it doesn't matter what character you use) when you activate your super move and defeat at least 2 enemies at the same time (double kill) the trophy should pop.