I fought Yunalesca three time, unusual for me. First time, Mega-Death caught me off guard. Second time, I tried being conservative because I had no idea how many forms she had. I held back, and she Mega-Deathed me again. At this point, I figured out that it doesn't hit Zombified characters. I intended to use this to my advantage, but never got the chance. Third round, I just said, "You know what? Screw it. Die, crazy lady, DIIEEE!!!" Long story short, I went all out and killed her - she never even used Mega-Death. I'm still not sure why.

Also, RikkuYuna's right. Holy magic make Yunalesca crawl into corner and whimper.