I did kill one or two lonely guards, using trance and then sneaking up from behind and killing them, but other than that, I was hiding and using the vents to get to the briefcase. The hard part was getting out. I thought the two sides would start shooting eachother, but they just stood there doing nothing. Wich made it difficult to escape unoticed, and I was spotted when I was at the top floor, so I just made a run for it.