If you’ve been struggling on this one (she can be pretty tough), here’s a walkthrough that will hopefully make things easier for you.
First off, this is a fight that rewards the patient and well-prepared. The Forbidden Forest, which is just before this fight, is a fantastic place to grind Blood Echoes to boost your stats, as well as heal pots and even blood stone shards for upgrading your weapons. I recommend grinding a bit before tackling Rom, and putting some extra points into your stamina will make this a lot more manageable, since you can string together longer combos. (That’s actually true of most the boss fights in this game, actually.)
Also, I like to equip the Communion Rune, because it increases the number of heal pots you can carry, and I like to have the option of healing a lot in case I need it.
This fight happens in three phases, and each one starts with Rom summoning a wave of broodlings. You’ll be tempted to ignore them and just start slicing into Rom’s meaty, succulent body, but I would advise against this. The broodlings will start stacking as you progress through the waves, and leaving them alive will make this fight exponentially more difficult.
Instead of jumping right into the middle of the broodling swarm, I advise keeping to the edges of the swarm so you can pick off broodlings one at a time as they get separated from the rest of the pack. These little guys are unpredictable, and they spew streams of poison, lash at you with their creepy little legs, and even dive bomb you. When you try to take on five or six at a time, it becomes very difficult to watch for these attacks so you can dodge them.
Oh, and keep in mind that, just like Rom herself, the broodlings have armored heads with squishy bodies. Attacking the body does far more damage than hitting them in the ugly faces.
Anyway, for the first phase, Rom just sort of hangs out. Kill off her broodlings one at a time and then attack her. It’s pretty straightforward.
In the second phase, she’ll start tossing ice shards at you, so you’ll have to dodge these. You’ll see her start to glow and lurch up a bit, and when she does this, start rolling as quickly as you can, even if this means putting some distance between yourself and the broodling swarm. (You can always make up for lost time, but your health is limited. It’s far safer to waste a little time than to waste a chunk of health.) Once you clear out the broodlings while dodging the ice shards, run up to Rom and attack her squishy sides. She’ll do this thing where she rolls onto her side, and when she does this, leap away as fast as you can; she’s about to do this splodey, icy thing that’s going to hurt you a lot.
The third phase is just like the second, only Rom attacks far more quickly. She’ll also do this jiggly thing where she shakes as you get near her. Be careful, because getting hit by her mad jiggle will damage you.
And that’s that. It’s a pretty simple fight as long as you try to not get too far ahead of yourself and spend the time it takes to wipe out the broodlings every phase.
Here’s a video of the fight, in case you want to see it for yourself: