You deserve some props for making it to this particular fight, since the only way to access Rom, the Vacuous Spider is to essentially commit suicide. Granted, you won’t actually die when you jump off of the edge of Byrgenwerth’s Lunarium balcony, but the distance between the leaping point and the lake below initially appears to be a source of certain death. It takes a certain amount of cojones to say, “Screw it,” be willing to sacrifice all of your Blood Echoes, and jump into that glowing white circle, and clearly you possess them. Unfortunately for you, Rom waits on the other side of this portal, and if you don’t know what you’re doing, you’re almost certain to die.
Like with every boss battle in Bloodborne, having an ample amount of Blood Vials and a powerful weapon will make things far easier for you. Even though its low durability can cause a few problems, I found that a highly fortified Tonitrus was able to do a massive amount of damage to Rom and his minions. Having some form of elemental damage buff on your weapon is going to do you a world of good, considering that this multi-stage battle can be exploited into being a bit shorter. Possessing the Old Hunter Bone is also going to help quite a bit, as Rom’s tracking aerial magic attacks are a bit difficult to dodge. Unlike the Shadows of Yharnam, the name of the game here is patience; if you don’t relax and take your time, death is essentially an inevitability.
After entering Moonside Lake, take a look around until you notice a glowing white blob in the distance (this can be a bit difficult to see against the bright white sky). Whenever Rom appears, this is the animation you’ll see, so be sure to keep an eye out after each stage of the fight. During each battle segment, you’re going to want to repeat a theoretically simple process to give yourself the best chance possible: kill all of Rom’s minions while dodging its attacks, dash around to the boss’s backside, and unleash as many high-powered melee attacks as possible. Being conservative will make this fight far simpler, so don’t worry if you feel like things are taking a long time.
During the first segment of the battle, Rom will simply spawn a horde of small spiders that will slowly make their way towards you. Like their master, these spiders have a hard, damage absorbing skull and an incredibly weak body. Making sure to avoid any spiders leaping or swiping at you, dash around to the side of any given target and unleash a melee attack. Assuming you’re at a high level, one or two of these body shots will surely take out a minion; simply repeat this process until every smaller spider is defeated and move towards Rom’s sides and back to deal some damage. Having a high-powered elemental weapon will help here since Rom will eventually disappear after its health bar hits a certain threshold, though you can move past this trigger point if you’re able to hit him hard enough. It’s also worth noting that you can still attack Rom during its disappearing animation, so when you combine this with the massive damage you caused before, it’s entirely possible to make this a two-stage fight as opposed to a three-stage battle.
The next portion will see Rom spawn two hordes of minion spiders and begin utilizing magic attacks. Whenever you see the massive spider raise up on its hind legs, be sure to begin dodging or side-stepping (preferably using the Old Hunter Bone) in order to avoid the homing crystals raining down from the sky. If you’re far away from Rom, one of its barrel rolls will cause a massive array of crystals to emerge from the water below you, though these are rather simple to avoid. If you’re near him, these rolls will cause a massive sphere of magical discharge to damage everything inside of it (this can be avoided with a simple roll backwards). If you’ve been doing enough damage, one or two of these hits will only take off one or two-thirds of your health bar, so using Blood Vials can work just as well as dodging. There’s not really much strategy here other than dodging its magic attacks, as the damage dealing process is exactly the same as before: kill the minions, dash to Rom’s back and deal damage.
Depending on your strength, stamina and weapon choice, either two, three, or four of these stages should cause Rom to die, granting access to a bizarre cutscene, a Kin Coldblood and entry into Yahar’ghul (the toughest area yet).