I'm pretty sure last hit is random. I've even changed formation in the past to do more damage on last hit...makes no noticeable difference. I got last hit on cerb and was #1 in banish plenty of times, so low power don't matter from what i've seen. One of my guildies got last hit like 3 times in 2 days. I even got last hit once after I left...hit him once then went to work on gear.