who to pull firstFollow

Casters and Healers alike they must be pulled to prevent adds. To do that depends on group and surroundings. Healers die first btw. 1. Improved Counterspell(Mage with Arcane Talent) forces the mob to run to you until a) melée range, or b) you start ranking it. 2. Silencing Shot(Marksman Hunter) same as Imp. Counterspell, different Class. 3. The Famous 'Corner' pull, everybody starts behind a pillar, corner, doorway, while the tank runs out shoots healer/caster and runs back to the group LoS pulling the group of mobs back to your group. 4. Earth Shock, a Shaman follows the warrior, warrior shoots and demo shout or thunderclap to get group aggro then the shaman runs to the caster, earth shocks, then runs to the warrior to pick up off him/her. 5. CC the caster kill the melée then kill the caster.

Casters try to put ranged attacks on you. You must counter the distance. You can either silence the mob - to force it into melee mode, or break its Line of Sight (LoS).
The corner pull technique (Shoot it, and run behind the corner / wall) will bring it to you. Running away from it will also gradually bring it towards you.
If you are within melee range, a Shield Bash will also silence it and force it to melee you. Pull it back. Your mages can counterspell it. Your hunter can silence it. Your rogue (at a pinch) can Kick it and Vanish.

Your strength, as a warrior, is in tanking the melee mobs. You can do this better than anyone. You should be picking up the runners, holding them and pulling them back to your killing field. Let your DPS deal with the stray caster, either with violence, silencing spells or breaking line of site.