This Seeking Tech guide will explain how you can defeat The Girl at the Water Treatment Plant in Resident Evil Requiem.
You can check out how to do so, while only needing to use minimum Requiem ammo, in the video and text guides below:
When you get to the lower floor, make you way first to the Chemical Treatment panel. Before interacting with Crank Lever, pick up the Molotov and then equip it. After trying to turn the lever, quickly turn to the right and throw the Molotov at The Girl before interacting with the lever again. If you time it right, you should be able to power up the Chemical Treatment panel.
Next, you should head over to the Control Room panel. If The Girl ends up blocking your path on the way there, run back to the Chemical Treatment. She will most likely drop down once you get there, opening the path to the Control Room.
In the Control Room, head to the small storage room to pick up the crank lever. If The Girl is chasing you, wait for her in the small storage room to evade her by using the middle shelf that divides the room.
If she isn’t chasing you, then she will appear when you head to the control panel. In this case, fire at Requiem at her after you recover from being thrown. Afterwards, quickly use the Crank Lever to power up the area.
For the Storage Area, The Girl will most likely be blocking your path. Shoot at her with the Requiem to stun her and then quickly rush over to control panel. If the boss pops out at the control panel, then there should be a spotlight nearby that you can use to stun her. Of course, you can also simply shoot her with your Requiem.
Once you lit up all three panels, the power should be turned on. You can then backtrack to the elevator. Once you return to the top level, quickly press the button to open the roof. You may have to shoot at The Girl if she ends up blocking your way.
Once you open the roof, the fight will be over. Despite the game telling you to shoot, you actually don’t have to waste another bullet as the light is enough to kill the boss.