I found the easiest way to do this, as I took the skiff and had trouble getting guards to follow me and then get knocked off, was during the last mission.
Sneak your way towards the entrance to the eye, and leave everyone alone. From the stairway up to the three cultists talking about books you can foresight and then displace into the top ledge. Turn back to face the library, you can displace into a ledge in that bookcase right in front of you, walk to the edge, peak around the corner, and give that cultist hanging out on the railing the old what for.
It took a moment to pop, but I was able to get it on the first try.