By now the WoTLK Classic Gold second archmage should have been revealed to you on the map, so you'll know where to find him. You can actually use the same tactic again to kill him, if you wish, but you'll have to deal with more support units this time, so it's a good idea to have some combat units accompany your heroes to run interference while they deal with the wizard. Once he's dead, find another cage just above him that contains an ogre magi. Now head back to the fountain of health you passed on your way in and heal up in preparation for your next assault.

Antonidas' entourage is substantial, adding to the problem of the holy aura.

The second town is to the east of the fountain of health, and it's a little more heavily defended than the first. You'll have a fair number of knights and sorceresses to deal with, so stock up on crypt fiends and abominations if you're low. Meat wagons are also invaluable against stationary defenses like towers, of course. Burn this town to the ground to stop it from sending assaults against your own base. By now, you'll probably be visited by some gryphons that spawn from a town you can't access, but these will be destroyed by spirit towers if you've built enough of them. Once the blue town is gone, you're ready to go after Antonidas.

You should definitely have a fairly large force ready to come into the last section of the aura with your heroes, as Antonidas is surrounded by quite an entourage. Again, have your heroes make a beeline for him and let your teeming mass of melee units intercept the fighters nearby. Even the much stronger Antonidas will fall to the combo buy WOW WoTLK Classic Gold of death coil and frost nova if you press him with it. Don't worry about trying to kill all his defenders--once he's dead, the mission is over.