Let's Join the KISS: Psycho Circus - 22 - Midway and Stargrave
"Midway and Stargrave" sounds like a law firm, doesn't it?
It's right before the three-quarters mark of the game and they're still adding enemies to catch you with your pants down. Not complaining, though. It keeps things interesting.
Stargrave is actually kind of tough. He drops those balls of balls of electricity that act like proximity mines, has a lightning attack like the Arachnoclown only automatic, and to top it all off he's one of those "break these things around the room or else he's just going to keep reviving himself" bosses. Keeping off the central floor as much as possible is a start.
Honestly, if I didn't know any better I'd say they designed the boss arena specifically to make it difficult to use your ultimate weapon on the towers, just so you couldn't cheese this fight with it. Difficult, but not impossible.