I FINALLY cracked the enigma that is Death's first phase. I never managed to find a consistent approach, and his actions seemed to be nigh impossible to determine, or affect, for that matter.
It turns out that I was kinda lucky in my first run, and decided to jump up and hit him very early without having him stay up in the ceiling. I tried to replicate this both in this run and in my any%, but failed to do so. But it turns out, you have to have the attack him as soon as possible, then wait 'X' amount of frames, then continue to attack. Then he will swoop down extremely low, which allows for a very long time to attack him without delay. I'm not sure what the difference between this run and my first is, but somehow I manage to save nearly 160 frames in this fight, which I find really amazing (my first take on Death, before I pegged this trick, only ended up 30 frames faster and considering I entered the pausescreen in my first run, the actual fight ended up slower).
Pie
Nothing special during phase 2.
1133 frames ahead of my first run.