If you want your compainions to focus their abilities, simply set them to attack the same target as the PC. I will tell you that often, this is not the best route to take.
My own strategy is generally to pair my tank up against the deadlest enemy I can find and attempt to drag as many foes to him as possible. The healer can thus focus on healing the guy who is going to take the least damage and the highest bonus health with healing percentage. The remainder of my party (the DPS characters) focus on taking out the secondary members of the opposing group. If your tank is properly holding the enemies and suitably equipped, you'll find few enemies that really prove a challenge. Obviously you have problems if you try to send your tank toe to toe with a few of the deadliest enemies in the game but I'm sure you can develop tricks of your own to defeat such monsters.
My own strategy is generally to pair my tank up against the deadlest enemy I can find and attempt to drag as many foes to him as possible. The healer can thus focus on healing the guy who is going to take the least damage and the highest bonus health with healing percentage. The remainder of my party (the DPS characters) focus on taking out the secondary members of the opposing group. If your tank is properly holding the enemies and suitably equipped, you'll find few enemies that really prove a challenge. Obviously you have problems if you try to send your tank toe to toe with a few of the deadliest enemies in the game but I'm sure you can develop tricks of your own to defeat such monsters.