Yeah, I had a problem with that one too.. your best bet is wait and draw it close to the door, then when the cannon comes out, block shots until only 1 archer is left.. since if you kill them all, they respawn after a certain amount of time.. just leave 1, which isnt nearly as bad.. then beat it down.. I used a combo of magic and melee. The best time to melee is when the battering ram comes out.. and to avoid it, just run back towards the door as much as you can.
It's tough, and takes a while.. but I think right afterwards, you get a rank up and more moogle stuff. Also, your best bet for a weapon is either one with the highest possible strength, or one with decent magic and strength.