I don't recall that mission in particular, is it a side mission? The main storyline ones weren't too hard as I recall, but some of the side ones were brutal. There's some tricks you can use 'cause the AI is kind of dumb (like resurrection/quickening loops, running in circles around the map).
edit: Ah yes it's the 10-level dungeon to power up Vaan's weapon. The main trick you use is to give all the summoned creatures to Basch, power up his quickening (to make his entire group invincible), then send him in raging. Hopefully he fills his quickening again before it wears off, so you can chain them together. Otherwise just rez him and run away and repeat.
That's the last side mission though, and while you'll need to be probably 70+ for it, if you beat Bahamut and Zodiark already you shouldn't have any trouble with the main storyline missions (can beat at 50 pretty easy).
