well when i first was fighting it yuna was lv 54 rikku lv 55 paine lv 55. tried berserker and darkknight and i still couldnt take it down i even use yuna's special dress sphere and it didnt do much
I actually was lvl 56 when I first tried to play him, and yeah it isn't easy at all. Leveling up is always the way to go for this part and it'll make things easier come much later fights.
If you've unlocked "Cid's Secret Dungeon" in the Thunder Plains its a niffy place to lvl up a bit, but for him I recommend at least being lvl 70 for Yuna.
And try using Paine or Rikku's Special Dress sphere as they'll be the lvl higher than Yuna.
If you want to not use the Special Dress Sphere's then put Rikku and Paine in as Dark Knights and Yuna in as a White Mage. Her constant White Magic effects like regen, protect, shell and cure are handy to keep your dark knights alive.