×I have tried everything I can do, but I still can't do this.
Obviously, I should use the keg to strike the orb, but the slayer always avoids to blow up the keg. As far as I know, if the slayer needs to open a door, he will not avoid the bombs, but when I tried to block myself using mirror, he will break the mirror. I attempted to detonate the keg by myself on the door when the door is closed, but the slayer can't survive to the northwestern orb.

×Standing on the door and getting the Slayer to close it is the correct approach.