The only way I managed to beat him was by summoning a friend. Rom is fucking lame. If you REALLY need to go at it offline, the best strategy seems to be in phase 2 and 3, kiting the smaller spiders away from him--enough to leave an opening--so that you can dive in, get a few hits in, then bail out. It's a real test of patience. If you just take a couple pot shots in here and there, he'll eventually go down. Throw some high Arcane resist armor on in case his ice attacks get you and hope for the best.
Also, bolt paper. Shit does pretty heavy damage.
But yeah, fuck Rom and fuck spiders.