I run the same comp, prot pally holy priest, and 3x mages. My mages are specced arcane. I did some pugging for gear individually on a bunch of my toons and am now equipped in half 10/25 naxx gear. I also had trouble with the sladran fight. I did down him once (got the achievement for not getting snake wrapped on everyone but the tank) I haven't been back since I got my gear upgrades, but my DPS is double what it was before, and my heals are just huge by comparison. If I start the fight out with icy veins, arcane power, mirror image, trinkets, etc, I can easily pull over 3k dps on each mage, and probably bursting up to 3.5k. If the fight is short enough I could probably just spam Arcane Blast until I go OOM.

So yeah, frost is old hat for max dps now. Go arcane, use the rotation Arcane Blast, arcane blast, arcane blast, arcane missiles, arcane barrage (I use a sequence so if one mage procs missile barrage that mage keeps going on the sequence, so after a while they are all out of sync) I have also done it manually, which can increase dps, but if you arent careful, will run you OOM. I use mana gems every cooldown, and use evocate as often as I can. Arcane mage max dps is all about dumping your mana. With all the buffs your team has, you should last long enough to down the boss with relative ease.