So, I've done some messing around with the TAS tool on FCEUX for this game trying to understand the RNG involved and if there are any ways to manipulate it to your advantage.

From my findings, I would say that it's pretty much impossible. In setting up the first boss (belt) for the optimal pattern, I found that the FRAME you take out the mob on the ladder before going into the vial room, will change how the boss acts.

The final boss seems to be even more random. Whether or not he stops and laughs (turns red and vulnerable) changes based on how you move and if you shoot a fruit. It seems that turning your back makes him stop more often, perhaps - it's extremely hard to tell.

In addition, drops/demon spawns seems to be based on a 6 frame or so window, and the game requires at least 2 frames between any actions.

I'm not sure if this translates to the Genesis version of the game or not. But if anyone who is more versed in HEX and understanding the works of games would take a look at this, that would be great.

I realize that I may be late to the party here.... but this type of thing does interest me.... I have been digging into the Genesis version, but I doubt that this should be significantly different and I have done some initial testing on the NES version using my findings from the Genesis.

From what I can tell it looks like there are 3 registers that update every time there is an RNG call. The enemies that do not have a patterned movement cause an update to these registers every time they move. Also whenever you kill an enemy that does not have a guaranteed drop, this also calls the RNG. The RNG values for the first few rooms seems to depend less on the frame that the enemy is killed, but rather the position they are in when they are killed. I know since these enemies move at a constant velocity in this room, the two are effectively the same...

I have TAS'd this theory out. Both waiting before entering the room, as well as freezing the memory address for the enemy and preventing them from moving. And in both cases I was able to get the same drops and the angel to spawn before the first boss. I have also done some save-state practicing on the NES version and been able to notice that depending on what the drop of the enemy in the first ladder room is... I can change where I kill the second enemy to give me a higher chance of spawning the Angel. I figure with more practice and consistency one could get it more frequent, maybe every time.