It depends on which snake woman you're talking about, as there are several. The common thread between all of them is the ability to apply the Frozen effect instead of being able to just stun them and no immunity to the Burn and Harsh Chill DoT effects. The Ancient Serpent also has no immunity against base stat debuffs like Defense and Magic Defense lowering effects. I tested the Crossroads update with Esthera, NaN, and Kushiel in the party so it is possible to down them though using Jin and Pierce for tank and healer may make things a lot easier depending on your party level.