It's easy, just tell them the priority list you want to take down first.
For me I tell them to always kill Priest first, Rogue 2nd, Mage 3rd, Hunter then Footmen.
So
Priest > Rog > Mage > Hunter > Footmen, that way, I know they'll be going for Priest first, if theres no Priest, then Rog, etc.
Or if Priest is last one to get there, then attack the current highest priority there, then depending on how much HP they have left, switch to Priest or finish them off.
H HoR is easy if you have coordination.