This game doesn't require that much grinding. By the time you start having trouble in normal mode, you're strong enough to beat the first few levels in hard mode. That way you will maybe have to re-do two levels.
On later levels, it's good to farm gold in the goblins' level. There are so many enemies, even if you don't beat the level and surrender you'll still get a on of loot.
For the last stage in epic mode, just spam time stop. You'll either beat the giant or destroy the castle.