Two Options

The Real Way
The Cheap Way



The Real Way
Get two archers, one tank(can be dpsive), and two paladins
You can also work with 2 archers, 2 tanks, and 1 paladin
If you must you can settle with a pure mage.

Have the tanker(s) equip both the damage and armor potions. Have ONLY him run into the field, stun, gun, and slash and move the boss towards the barrels. Try pushing stomping, or "scaring" him away into a barrel. When this happens, his mana shield should fall. At this point, both archers should be coming in, buffing, debuffing, and blast shotting the space out of the overlord. Quickly run back out. The enchantress(es) will always stay back behind the gates, and when someone dies, run in there, resurrect them, and run back out. Enchantresses SHOULD NOT be fighting!
If you have a good tank, he should be able to last the whole time without dying. It takes about 2 damage pots and 2 armor pots, as it is a BIT more than 5 minutes. Basically, the job of the tank is to do minimal damage and bring mana shield off. The archers to the giant damage, tank takes back aggro, and enchantresses just revive the fellers. This works very well.

The Cheap Way
So you don't want to or can't spend any money? Get a group of 4 tanks and a paladin. Tanks can tank. Paladin rushes around rezzing like crazy. Likely, you will all die, but at least you had a slight chance, and you didn't spend any money, right?