I would say Big crystal difficulty is 3, because you can very easily kill the bombinos with 1 bomb and after that even if you don't remember which one is the 3 and which is 4, you have 50/50 chance to get it right.
I would rate the design less, because there is no background and there is a

. A good level is not necesserily the one that has everything in it - red button, level, bomb changer, spikes, teleporters...