Just a quick reminder to use the pulse wave and to use it often. It not only slows downs enemy projectiles caught in it, enemies that are also in its range take INCREASED damage! You can tell by the green debris that flies out (showing that the "ducks" are not really yellow...).
If you're holding off from using it and running out of time on bosses, then you're playing for score, which requires much more accuracy and skill. If you're playing for survival, keep using it and watch enemies pop quickly.