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Load Save 3. It's right before final boss and they both have infinite rocket launchers. Billy's health got glitched so i cant raise it pass Danger with the editor. Sucks a lot but the only other save I found for this was broken.

You can take 1 acid attack from the boss without dying. When Rebecca gets on the valves immediately run directly across to the other side and shoot for its attention. when shes done try to run back horizontally to where you fought it's first stage and distract it. from there it's about trying to keep your distance and using the walkways to stay away from it. Also rebecca is full health so she can take a few hits from it as you try to maneuver around without taking damage