I don't remember that exact part of the game, but I remember the part with the green sphere that is mentioned after the section you bolded.
When I first played through the game, I managed to kill that guard (whom you are supposed to kill with the green sphere) by bouncing one of his fireballs back at him, and managing to time it so that the fireball went through the doors and toasted him!
Then after leaving and re-entering that screen, there were shards of glass from the sphere on the ground, which I couldn't figure out until I found the room with the spheres and one was missing.
This was a great game, it was probably the most fun I had with my A500 to that point... it was a bit short though.
I remember seeing it first on the PC's at school, and there seemed to be a few scenes in the PC version (maybe they were cut scenes?) that I didn't find in the Amiga port.
I think I'm going to have to load up UAE and play that level .. I'll see if I can get past the part you're stuck at
