If you are auto-resolving, my guess is there is a "Banshee" in the elemental's army. Banshees can only be hurt by magical means. Meaning, unless you have +fire/cold/lightning attack on your weapons (or on the actual unit) or unless you cast a damage spell on the unit, nothing will ever harm it. You could take an Epic army against a lone banshee and if you have zero means of magical damage the banshee will take them all down.
If you are not auto-resolving fill us in on some more details of what you are doing or what is going wrong for you.