Hello Trophy Hunters,
This trophy pops after you beat the boss on the 100th level. I personally did this with Ragna "The Bloodedge", as he is the easiest character for beginners to use. His combo chains are easy and do a decent amount of damage.
You unlock items to put on your character when you fight specific characters (usually their unlimited form) To boost Ragna and make Abyss even easier choose attack up items.