Rezo freezes in surprise and confusion when Zelgadis speaks, then relaxes again once he registers the question. Unfortunately, he doesn't have a positive answer to it.

"No," he says, "Not as far as I am aware."

Admittedly, he hasn't exactly been going around directly asking people "Hey, can you cast a Giga Slave? It's the one known spell that can destroy the Dark Lord sealed inside me."

He'd ask Zelgadis if he's had any success except it's fairly obvious he hasn't, otherwise he would have said something.