I've been at this for 2 nights now...!!! Can anyone give me a HINT as to how to get thru this nasty bit...?!?!?
Unclepine
It's important not to use to much time to establish a sniperposition upstairs. I did the first time and was not able to kill all before "Mission Failed" appeared. After backtracking some saves and rushing upstairs I had ample time to kill'em all :-)