Damn, decided to play it again, got up to lvl 22, had a couple diamond (torquise colered) gear, the game said my game was saved after the boss fight so I quit, come back to find it wasn't saved, can't bring myself to do all that again
lvl 16 was just too easy, just use Shield over and over and your units will simply bypass the angel and destroy the tower, giving you the win in about 2 minutes
I really hate games that let your cursor move outside the playing window, not only do you then have to concentrate on the game itself but also on where your cursor is