hmm exactly what part of rabbit hole are you having trouble with? I know the sentries are really hard to kill, but if u can get head shots that will put em down for good. A nice way of doing this is by crouching or hiding behind some cover, try to line up your cross hairs on the targets head, then crouch back, wait for the guard to pop his head out and quickly lean out and your cross hairs should still be where his head was and BAM hes dead.
get back to me if this doesn't help, i will log my game and try out some things myself to let ya know what to do.
this is a very good game...difficult..yes...it really challenges u. love making those head shots.