Not sure how difficult this is with controlling 5 since I dual box most of the time but when the boss goes for his shield I always park my toons right at his feet. That way when he does the cone of fire you can run to the other side of him without moving very much. At worst I have gotten 1 tick of the damage.

When I use my tank to dual box I always
shield - go to his feet and point my toons for movement for when he does the cone
maces - run quick and kite with the tank (any kind of speed boost is very helpful when getting distance here) and nuke with others
dual swords - blow cooldowns

Like I said I am sure it is probably much more difficult with 5 but that is what I have always done with 2

Hope this helps
Twins