I beat yunalesca on the first go but i had a tactic. Always have at least ONE character as a zombie. This way, mega death won't kill all your characters. I made sure i had yuna as the zombified one as she could use full-life to revive the other two. Try before hand to get your summons to have their overdrives. this way when they get their one attack, its a BIG one and can take away a lot of damage. If you have bahamut, his overdrive should destroy the first form in one hit if you've trained yuna enough.
Trust me, this way always works for me
