This trophy is titled "Victory Is an Illusion" and it is for defeating "Unlimited" Rachel in either Score Attack or Arcade mode. Unlimited Rachel makes full use of her range of electrifying plants and small creatures at her disposal. Naturally, playing arcade on Easy and attempting to fight her is the easiest path to doing this I'd suggest holding off for the completionists as you'll have to fight her while trying to complete Score Attack regardless.
If you are facing her and strugging, use Iron Taeger and abuse his right stick flicked towards the right lunge paired with his up on the right stick grab and you should be able to find a rhythm in which to defeat her. She's hardly an easy opponent, so I'd suggest having a second controller there to save you...especially in Score Attack.