This battle can be really challenging since the gaps and doorways that you can go through can be incredibly hard to see. If you’re having problems navigating the area, consider turning on High Contrast Mode to see where you’re going and to easily pick up various ammo and crafting materials scattered around.
This hulking mess of fused infected is dangerous and can take a LOT of punishment. Your main tactic is to keep your distance while using pipe bombs, shotgun shells, and hunting pistol rounds to deal high damage to it. The boss can easily break through walls which will also give you more escape points. Keep lobbing your pipe bombs to it, especially when it is emerging around the corners or trying to break open the doors. Try to land some incendiary shotgun shots at medium range then prepare to dodge and run away once it gets into melee range. The Flamethrower can be used but take note that this will not slow down the boss and its limited range means that you’ll be dangerously close to the boss when using it.
If you need to craft more pipe bombs, run away as far as you can to craft more. Keep repeating this hit-and-run tactic until you deal enough damage and force a smaller Stalker mini-boss to break away from the main body. This annoying miniboss will always attempt to flank you and will most likely follow the main body closely. However, focus on taking out the main body first which will force the stalker enemy to flee.
After it flees, you’ll have a brief respite. Craft your consumables, reload your weapons, heal up, then loot the ammo and crafting materials in the area. Once done, squeeze through the barely-opened door and follow the stalker as it climbs up the vent. Continue through the vent until you end up in another part of the hospital where you have to fight the stalker.
This stalker will be vulnerable for a few seconds after landing so use this chance to land some shotgun shots up close. After that, the stalker will attempt to disengage or throw a toxic projectile (similar to the bloater’s “bombs”). Keep your eye on it and if it attempts to hide behind cover, throw pipe bombs in its immediate position to let the massive splash damage do the job. Finally, after killing this boss, loot the items in the area then open the shutter to the garage.