@qwak: actually the boss levels are kind of easy when you have the right technique (egypt stage and works even above)
Destroy most of the vanishing tiles and the blue monsters will be stuck just in front of you at the bottom left or right of the screen. Then shoot as fast as you can and you have some extra seconds to collect the loot.
Without that trick, the egypt level guardian stage is reaaaaally hard.
I don't have an iphone just a lousy ipod so I wish you good luck with your app.
(but I already bought it for the Amiga, for the GBA and for the PC!)