They should drop from the second boss of that disc. You can very easily end up fighting the final boss before that spawns.

oh, thanks, and yes that did happen, I typically have fast fliers uncovering the map pretty quickly followed by lair killers so they tend to find and destroy the boss lair pretty quickly. Guess I gotta remember to keep my lair killers away from it at least until the 2nd boss shows up.

Titan Slayer parts drop from the He-man like Titan on the 3rd disc (appears in the second wave of titans), if I recall correctly. You get one part per run, which is annoying

Hello! I've beat all the discs including chaos mode, only achiev missing is to spawn TItan Slayer! Incredible, I've beat the 3 disc (before Death was a thing) and never saw him, the He-Man guy!
But today I made a Divine Titan for the first time (jotun/angel/demon parts - divine) and the He-Man spawned on the 2 wave of 3 disc for the first time. After that, Death comes and end the run.

So it's really that boring? Only one part of Titan Slayer per run? It's really that bad? I'll be danmed...

Once the main boss has been beaten, the Adam Eternia mini-boss spawned next time on the 3rd disc. He continued to spawn with boss waves until I had all the parts (had first from first time facing main boss).