Use tonitrus, charged with L1 before attacks. Should kill the little spiders in two hits.
First, kill the little spiders
Then attack Rom on the side of his body (i.e., not his head) with the charged tonitrus. You should be able to get him down to about half health before he transports.
From that point on, ignore the little spiders and focus on Rom. You need to look for openings to sprint through the spiders to hit Rom in the side. If you see spiders jumping at yougfto of there. Same if Rom rolls over. Try to stay close enough that he doesn't use his rain attack. I killed him before he teleported a third time using this method.
This. This was also the turning point for me where the game got really soul's like and I hit my wall at this point. Once i did this Rom was a joke and killed him after two tries. The smaller spiders are no joke.