Having just taken the battle tower, I went back to Broken valley for a quick peek. The opposition there is at level 27. I am level 21, and while I managed to blow the airborne forces out of the sky, once I landed they made short work of me. Talk about pining for the fjords.
As a level 28 mage you should do fine.
Oh, and you might want to go in by air.
The valley floor is wrapped in a poison fog.
