Smash Party - Hero Action Game (World 8 Final Boss) - Tips?

I found an offer for this game on Adgate offerwall (GG2U) with multiple step rewards. One of the final steps is to complete the last world (world 8) on normal mode. Well, I seem to be stuck at the world 8 final boss. For what I can see only the weapons and buddies can be upgraded, which I have almost fully upgraded (except the store ones). can anyone who has completed this level share any tips?