My own worst enemy was those damn pistons in level 15
...and the fact that your own shots can obscure the boss's, especially when they hit part of the arena
The final boss is pretty cool, though, as is the creativity of the other bosses (even the damn pistons). Later games even make callbacks to this one, like "Big Eye" from Nekotosakana is a callback to level 13, and another game's final boss is a callback to the final boss here (especially with the music)