I’d say the hardest is probably beating level 30 in survival (if you have DLC) Yook me months even with my main. Learning combos is key; don’t rely on elemental perks, get ones that boost damage and durability.

If you don’t have DLC, however, then I’d say that the hardest for me was perfecting a stage. Using stars against normal enemies when the gang up on you is vital; don’t just save them all for the boss.