[Dragar]

There is a counter to everything, but there is a difference between being susceptible to a swag of mages with gear, and being a sitting duck for one astral mage

A group of astral mages is your biggest nightmare as utgard, got to find a way to be creative. I have that issue in a current game, there are no simple solutions and it is very painful. If you have decent earth somehow use rain of stones or earthquake on them, or fire storm if you have the fire. If you have blood high enough use some devil thugs or drop Infernal Disease on him. Cast leprosy. Bring your own astral mages and Banner of the Northern Star and mind duel if you have a heap of seithkona handy and are willing to accept the losses. Possibly form a communion and have the masters mind duel.

Failing all that attack where all those mages aren't, and remember that if he has a heap of mages out fighting they aren't at home researching or site searching. If an opponent can really play on your weakness you'll have to be more patient.