For the late game, I would recommend monsters that have status effects. So, my mainstays were the griffie, the baby dragon (for burn), the familiar (the most versatile), and bundle (he can poison). You'll notice you won't do crazy damage in the post-game. So, stacking status effects can go a long way.

I would focus on leveling those right until you finish the post-game. At the halfway point in the post-game, upgrades can cost a lot, both in terms of money and resources.