My tried and true formula is to kill the little spiders (they usually only take one or two strikes) and then focus on Rom. Tonitrus + Beast Blood Pellets can make short work of the dumb thing, but if you're not set up to take advantage of that, it's just a matter of recognizing which of three moves Rom can do.

Also, there are two NPC summons available for the fight, which can help with management of the little spiders, but two summons does cut into DPS quite a bit.