I'm a touch confused by the current setup. Namely which way is the archer, and how high up. So a few clarifying questions:
How high is the second floor of the tower? How high is the roof of the room where the group spent the night?
Which way is the archer dangling? Over the room where the group slept? Down-right outside the keep proper? Up-left outside the keep proper?
If it is possible for Siobhan to position herself so that she can catch the man in case of a fall, she'll move there. Otherwise move to engage the remaining skeleton ASAP