Actually for me Yunalesca was pretty easy. You just need a method. The first time i played i did her about 5 times and couldnt defeat her...now its simple. Just leave your characters on Zombie. She can only life one at a time, and when that happens just life them back. With all the characters on Zombie the attack 'Mega Death' (which kills all chars) dosnt work. The only attack you really need to watch out for is 'Mind blast' cause it puts confusion on them (and we all know that confusion on Auron is definatly a bad thing). Thats pretty much how i handle her.....Oh and it pays to get the first form out of the way quick cause its the one that absorbs the HP so use grand summon Bahamut or something...just get rid of it. (I think the second form absorbs HP too so try and get rid of that aswell).