I use focus on my main. My alts use /cast [target=focus-target] DPSSpell for my damage and /cast [target=focus-target-target] HealSpell for my healing. It doesn't matter who has aggro, the heal still goes to the person whoever the mob is beating on. You can add in more later to make it fancier like /cast [target=focus-target, help] HealSpell incase you target a friendly person or a sepecific party member. And /cast [target=focus] HealSpell incase the mob detargets everyone (running, fear, etc).