Many people find it struggle when grinding for Lv99 Trophy (at least what i found in Gamefaqs discussion board) well indeed, but since the free DLC is up, it's quite easy to farm, especially in No Hope mode which gives 3x EXP & 10x Money but enemy is ridiculously stronk, so you need better gear to play in this mode. Out of few end-game beast, Prototype x3 seems to give the most high EXP, but killing them takes quite a while. But believe me, when other beast no longer gives anything good, Prototype x3 is the answer.
Note:there's freeze glitch when you try to save game after fight against Prototype x3, but it was fixed in patch 1.04 last week
