Re: volunteers.
Illyana was specifically not fighting in Sera's tunnel since Sera was all "AHHHH DEMON" about her (even though Illyana would have liked to troll Sera, but during battle isn't the best timing). She's generally an up-close fighter, but with that Despair Demon being a ranged fighter, she wouldn't get much in the way of hits on it.
Since those barriers, on the other hand, are apparently vulnerable to dispel effects, she would be dispelling the hell out of them with her Soulsword. She'd end up fully armored for the battle to keep the ice spikes at bay.
Unless their side was already too crowded, Illyana would prefer to fight alongside the Winchesters :D