If you don't have this by the end of the game, the best way to go about getting it is to defeat 10 red crystal stages to unlock the Bone Crusher weapon in the store. Upgrade it fully, as it increases the chance of a Critical Attack by 4.5%, then grind levels. You can use Rose, who has an 8% chance of a Critical Attack as opposed to Grimm's 5%, but it's not at all necessary,