With a bit of cheese, it can be won! 😉 You can use the door as a choke point; leave one of your characters there, out of line of sight. Then use the character with the highest vitality and armor to go in and press the green button to free the lich, then run back out the door and nip to the side, out of the line of sight. You'll take some damage, but the lich should help and once you get out the door, you'll be able to bottle each opponent up as they reach the door. They'll use AOE abilities to try and hit you from afar but eventually they'll have to engage close-up and that gives you the advantage with two melee characters. Just stand on either side of the door, heal and buff each other up, and engage each opponent as it comes and you should prevail.
You could also use a summons as a decoy and let them beat it to death while you head for the door.