-During the first phase, focus on getting rid of the spiders first. Since spiders won't despawn when you move onto a new phase, getting rid of the first 12 will give you some more breathing room later on, since you really can't focus on them once Rom starts using his moveset.
-Take advantage of an old Souls trick: Rom is still vulnerable during teleporting, so you can get some strong hits in before he completes, and you can get him down to 50% or close to it during the first phase. He begins to teleport when he reaches 75% and 50%, so anticipate them do maximize free damage.
-Learn his moveset once he begins to attack, which are as follows:
1) Rom points his head to the sky: sky crystals are inbound. Roll to the left or right and you should avoid them.
2)Rom points his head and abdomen to the sky: close range AoE blast. His deadliest move, and my personal recommendation is if you're locked on, disengage immediately and roll away. The reason for that is the thing that can screw you up the most is not having camera control to roll past the spiders, which can block you in.
3)Rom rolls on its back and does a crunch: ground crystals are inbound. Pay attention to the water to see ripples to signify where they're coming from. Deadly, but easy to avoid, and it's possible that you can get an opening by Rom while its on its back to do some extra damage, but it's not likely, as the crystals tend to form a perimeter around his body.
4)Rom swings his head: he, umm, swings his head. Generally doesn't happen when you're attacking him normally, but he can go into it if he backs up and he's looking directly at you. Dodge to your right back over to his side and you should miss it.
-Most important tip of all: always be dodging. Whether you're locked on or not, keep moving that way to take advantage of the I-frames that will save your life more often than not. Rolling is probably the safest way to go about this, since you'll retain camera control so you're not running into the spiders.