Ah what I could recall is two rows of enemies which shoots bullets, one row a the top of the screen one tow at the bottom. I failed to destroy enough of them and the bullet rains was a disaster for me. Again, I was never good at shoot 'em up so maybe not a good sample point :)
One thing I can think of tho is some sort of invincible cool down interval after the play was hit so that the HP won't drop down super fast? (If there is already one then maybe it's too short and I just didn't notice)