Fractured Ward (Killer and below), shield as fight-item and a few fast, and in your opinion, good fighters.
Equip also a few bots for Dmg (+/-25) and maybe some other buffs. (I prefer to time the health buffs, so that they end right before Gargos comes.
I start with the badest character and end with the best one, so that he has a kind of bonus through an already damaged enemie.
Use the equiped item (the shield) first when Gargos starts to totally freak out.... you need to wait for it with your shield or it's mostly just a wasted character. This works also when Gargos has all buffs. (i've unlocked Gargos' buff-achievement with it)
btw: i haven't played with all characters atm, but i prefer Sabrewulf, Riptor and Maya (previously Jago) [or Orchid] because they have really fast attacks and in the most cases you don't need to block attacks.