I just adore the concept of going back through the rooms to find the same object in each one! This is the first time I've encountered such a design. Lovely game!
Step one: make your tank nothing but guns.
Step two: revolve around the edge of the bubble, shooting wildly in the general direction of the center.
Step three: ???
Step four: Profit! (Not really, just waste lots and lots of time.)
The store system is very frustrating. As soon as I get enough money to buy something, I level up enough so that the items I'm presented with are too expensive to buy. I also don't see the point in being able to upgrade specific skills if it's all randomized. It rewards being all-around.