Some really good pointers, thanks. Had no idea about the current health thing.

This specific Omega actually has a great Fandom page too. More things I didn't know: Meteor goes through Reflect, and, crucially, Meltdown is a counter to using a Blue Fang. So. Three guesses what I used before he killed me.

Using those two facts, I spent about 50 minutes and almost all my vital items, realized I could win the battle of attrition if I didn't miss a move, and never saw Meltdown. Finally got him Bring on Nova Dragon...