It's not really clear what you have to do to get 3 stars in a stage. It's not (but it should be) fewest shots. It comes down to keep grinding the stages until maybe it gives you 3 stars one of the times. Even times I do awesome and I get one star and then I play again and do poorly and 3 stars.
It's also not extremely clear how you get the various items. Is each collectible item hidden in one of the relevant types of containers, or is it a so much percent chance that blasting any of the relevant containers may give you a missing piece?
Grinding to get 3 stars in each stage and grinding out all the collectibles was not fun.