NDCP2 Map 22

Ok so I have the red and yellow keys and have found the blue key, but when I pick it up some doors close, and a switch is revealed, flipping the switch opens one of the doors.

But then there's an area with a load of barrels and the switch to proceed behind it. If I shoot the barrels all the Archviles teleport in and I'm toast.....how do I proceed without getting killed (apart from cheating of course !).

Cheers.

Phill.