Final battle dome: (Rebecca and Billy) (2)

Part 1: The Initial Assault
As soon as the fight begins, have both Rebecca and Billy unload all their magnum ammo into the Leech Queen. Don't hold back! If Rebecca takes too much damage, use an item to heal her. Keep up the barrage of magnum shots. You'll know you're doing well when a cutscene triggers. The Queen will try to hit you with acid, but it doesn't do much damage. The key here is to just keep firing and avoid its attacks. This part should go by pretty quickly thanks to the sheer power of the magnums.

Part 2: Valve Control and Distraction
This is where things get a bit more strategic. Rebecca needs to turn four valves around the arena, and Billy's job is to keep the Leech Queen off her back. Your main objective is to protect Rebecca at all costs. Keep pumping the Queen with magnum rounds to distract it. It might try to charge at Billy, which can be good for creating distance, but be careful – it can knock him down, giving Rebecca precious time to turn the valves. After Rebecca turns the first valve, let her escape the immediate area. Then, lure the Queen towards that first turned valve handle. This creates a clear path for Rebecca to move to the second, third, and final valves. Keep an eye on Billy's health; if he gets hurt badly, pause the game, go to the inventory, select 'Change,' and use herbs to heal him if he has any. Each time Rebecca turns a valve, the Queen will focus its attention on her. This is your cue to use the magnum to distract it, drawing its attention back to Billy. Try to bait it into a rush attack on Billy; this forces it to move away from Rebecca, giving her the time she needs. Once Rebecca has successfully turned all four valves, sit back and enjoy the ending cutscene. Completing the game this way will unlock the Rocket Launcher and Submachine Gun if you haven't already obtained them.