^You can get catnip assuming you did everything right in Bevelle for the kinderguardians to find the...umm..hole thingy in Bevelle.
But the way I beat this guy was level to
Yuna (71)
Rikku (72)
Paine (73)
Then have a mastered alchemist (assuming you got the alchemist dressphere) and 99 High Potions. When the battle starts, use a star curtain to cast reflect on the party, and the fiend on the right should kill itself. Use high level black magic and regular attacks to take out the fiend on the left. Then give the center one your all. If your health goes down, have an alchemist mix 2 high potions and that should heal 2000HP for all characters. You might want to enter this battle with a ribbon equippred or anti-poison accessories.
This is one of the most hardest bosses in the game (not counting the via infinito ones). If you're gonna climb down those 100 levels in via infinito, then dont use too many elixers or mega-elixers here.