Army of Ruin

This is around when the game starts requiring you to plan out your builds so you can evolve weapons so that you are able to survive until the end of the stage. If you're doing that, and surviving until the end, you should have enough time to reach Level 30 with each character on this stage before the boss fight. Personally, I was Level 33 with each character when I completed the stage. Focus on picking up gems and remember that these objectives and achievements don't actually require you to complete the stage, so if you have to grind kills at the end, do it even if it means you won't be able to beat the boss.