So as it turns out, here's a quick, slightly-luck based method for beating the Angel of Death. Works best as a Marine, albeit I first pulled this off playing Technician. Remember to get a 100% kill rate up to this point, in order to obtain Azrael's Scythe (play on HMP or higher difficulty!).
Begin thus;
1.) Obtain and SAVE as many phase devices as possible--especially the homing variant, up until Deimos.
2.) At Special Stage 7, the Spider's Lair, defeat all monsters and acquire the Nuclear Plasma Rifle (or obtain a Thermonuclear bomb or similar Nuclear item)
3.) At the stage preceding the Unholy Cathedral, do not take any God Mode power-up that appears. Protect said power-ups from explosions at all costs.
4.) Once the floor is clear, grab as many God-Modes as practical, and either run to the Red Stairs if they are close, or begin using phase devices until you get close enough to do so.
5.) Assuming you have ten moves worth of god-mode left when you enter Unholy Cathedral (Marine, again, extends power-up duration), immediately set off a nuke and use the wait command. This will kill everything on the level, including AoD (except you, if you're still in god mode), assuredly winning you Azrael's Scythe.
I've found that god-mode consistently appears on the level before UC appears. Could it be coincidence that they hand you a nuclear weapon on the bonus level just preceding this stage?