Buy 'Reeking Boxes', I think they are like $500 ea but they spawn Risen on the Map Area that you use them, depending where you put them the Risen are either strong or weak. I think if you use one on the earlier areas, it spawns weak ones. If you buy a few, you will be able to grind abit.
One thing to note about the Reeking Boxes is that the level of the enemies are in accordance to the area you spawn them in. Of course, you can buy the DLC map "EXPonential Growth" and just farm tons of EXP off of that, but that might feel like cheating before you beat the game. I use that map to level up characters post-game.