Wow. Um, that sounds kinda dim of the party. Was there a reason that they didn't just have someone stand in front of it with Protection from Evil armed? Or Magic Circle, to get all of them? That would have stopped the creature from approaching anyone who wasn't directly attacking.
Fly. Duh. A flying wizard with a few fireball spells could have handled the elemental without a ton of difficulty, and one Greater Magic Weapon on the party archer's arrows would have whittled it down.
Heck, off-hand:
Party Fighter: Accept every possible spell to buff up the AC and the hit points. Then use hit and run tactics to keep the elemental's attention. Use a pole arm to limit the ugliness of the reach battle. Don't get sucked into trading full-round attacks. Get Hasted by the wizard, and have your round be "Move close enough to attack creature, probably just a five foot step, make single attack, then make a full attack, and then use your hasted partial to get away." Focus on keeping yourself alive and drawing its fire, not on doing damage. Be happy with one attack per round, either fighting defensively or using Expertise.
Cleric: Protection from Evil. Greater Magic Weapon. Maybe a damaging spell or two if you've got 'em. Why wasn't he Flame Striking?
Sor/Wiz: Mobility. Get party members up in the air. Get party members invisible or AC-buffed. Above all, simply do damage to the creature. Its saves are unimpressive, and a barrage of fireballs or lightning bolts works great against a big, slow monster with no SR or elemental resistance."
Rogue: You can't flank, you can't sneak attack, and it's not really gonna fall for traps. Don't melee with it. Use ranged weapons as best you can. Tumble through occupied areas to deliver potions of healing as necessary. Hide or use other skills to stay safe, so that the cleric doesn't have to waste time healing you and the wizard doesn't have to buff you up. Honestly, aside from ranged support or momentary distractions, this is not a rogue's fight.
Ta-dah.
-Tacky