We killed him tonight on 10 man. I don't know if the same issue applies, but we definitely had a couple lock tank gibs due to Keleseth meleeing. Our lock just had to be careful to never get closer than 10 yards to Keleseth. And if he needs to run around Keleseth to get a buff on the other side, just make sure he knows to "swing wide" and not get any closer to the boss. Made the difference for us.
"Sometimes attaining the deepest familiarity with a question is our best substitute for actually having the answer." -- Brian Greene.