I'm very confused as to how and why I lost a

[Taqwus]

Unbeatable? Practically nothing short of an incredible investment in gems is unbeatable (defeating a horde of a thousand Tartarians under twenty decked-out Tartarian Titan mages would be ugly, but that's not normally a realistic scenario).

An Elemental Queen of Air, for instance, laughs at Storms, so short of paralysis, death or other incapacitation means there's no practical way to prevent one from flying over the wall to wreak havoc. Pazuzu might be the same way, never tried. Horrors, spring hawks, storm demons are the same way.

Nasty long-range magic could also wreak havoc on the backup; even mere Illithids can mind-bLast across the map with perfect accuracy regardless of storms, night and even blindness. And SCs are still KO'd by Magebane (100 fatigue per hit, unresistable; rapidly turns into ugly damage), always paralyzed by Petrify, and take massive wounds if hit by megadamage toys like Gate Cleaver or the Summit with the latter wielded by a strong commander or under Weapons of Sharpness, or the ever-fun if not particularly accurate Gifts from Heaven.