General tactic, stims and force powers run around to split them up so that you can take them on one at a time. Use mines and grenades to full effect.
If you have a save from before he fights them then equip atton with the best items you have so as to boost his attributes.
If you are still having problems, then maybe you levelled Atton poorly, in which case you are screwed - there is nothing else you can do except keep trying and hope that you pull it off.