This Seeking Tech guide will explain how you can beat the Blister Borne at the Sewage Facility without taking damage in Resident Evil Requiem.
To do so, follow the video and text guides below:
When you pick up the valve wheel, two zombies will spawn. Try to lure them back into the tunnel where you pick up the item and kill them off there.
When turning on the valve, you only need to turn it around 1/5 of the way before the door automatically opens. You can use these few extra seconds to shoot at the weak spots of the Blister Borne. However, avoid destroying any of the weak points for now. Otherwise, he may summon Blister Heads.
Once the door is fully opened, try to kill the zombie behind the Blister Borne with your sniper rifle. Afterwards, lure the boss back near the tunnel and then run to the open path with your shotgun equipped.
On the newly opened path, use your shotgun on downed spear zombie in the circular room and then on the regular zombie near the stairway.
Next, climb up the stairway and wait on the second floor near the broken barricade that you can drop down to. From there, wait for the Blister Borne to make its way to the second floor before dropping down. Kill any zombies that may show up beforehand.
Once the boss is on the second floor, jump down and then head back to the valve. Try to turn it all the way if possible. If you see enemies appear, especially the spear zombies, then try to kill them first before continuing to turn the valve.
If the boss shows up before you can fully turn the valve, then you can lure it back to the tunnel and then run back to the second floor on the other side. Once he chases you there, drop down and run back to the valve.
When the valve is fully turned, lure the boss near the tunnel again. This time, you can start bursting its weak spots. Just make sure you have room to run to the second floor.
You can use the red canister and throwable weapons to attack the boss and any of the Blister Heads.
If you get overwhelmed, run to the second floor to the new pathway that was created when you turn the valve all the way. When you reach the panel with the battery, the enemies will stop chasing you. You can then attack them with their backs turned.