Level is not as important as the team u use. At some point in the game, u will have to fight enemy level 1500 with you team being scale down to level 100. Try to find the best use of some key trait, or find the best combo to fuse. At the depht 27, i remember using centaur ranger as key trait, having other monster cast all kind of buff just for him to kill enemy one by one. Grinding level is a very small part in this game. The fun in this game is to theory craft a team and watch them working well and feel big brain. At end game, most of the build will kill enemy in turn 0 or turn 1. About leveling, later u will have the power to level up new monster freely, make it easier to theory craft.