To unlock the Become that which you fought against trophy in Batman: The Animated Series DLC pack for LEGO DC Super-Villains, you will need to reach True Villian status be collecting approx. 60,000 studs.
If you have the red brick multipliers unlocked from the main game, they will make unlocking the trophy much easier.
Once you have achieved True Villian status, complete the ''Mask of the Phantasm'' level and the trophy will unlock.