The thieves' guild are not battlefield rated. They steal, not kill. I think you want the Dark Brotherhood. Companions, that's what they do. Even if the Circle stays in Natural Form. Faralda would rock Socks, Aren too, but the rest of them are fairly squishy. Line it up by matchups:
Kodlac-Aren. goes to Kodlak. While the Archmage can swing an iron battleaxe in Dragonhide with the best of them, I got a feeling the Harbinger isn't even going to break a sweat here.
Skor-Mirabelle Uh, Skor. I've never seen 'belle break out, but I have seen Skor go all dual wildy in Wolf Armor.
Vilkas/Farkas-Faralda Ninya. The Steel Twins. The elves would be too busy bickering between each other, or at least Ninya wouldn't try too hard to have Faralda's Back.
Aela-Tolfdir. Flames, and Stoneflesh verses one of the best archers around, Ancient Nord Armor, sheild and dagger. I like Tolfdir, I do. Toast.
Collette would have no idea what was going on, and probably try to Turn Undead. No offense, regardless, if she got her act together, maybe she could heal the wounded.
Phineas, ok, he's pretty badass. Arniel gane too. I just realized the numbers here. Assuming Drevis ran away invisible, there's more Professors than circle members, and the Apprentices can keep the trainees busy. Even strength, Companions. With numbers, and a wide open battlefield, or home team advantage, CoWH.