[Numahr]

If you have enough territory, you can play attrition: accept to loose the battles where he gathers like 10 mages, but take as many marshmasters with you every time with cheap tactics (archers, assassination, flyers, magic duel, etc.) or battlefield globals such as Earthquake (A Troll King can cast it), Grip of Winter (a Water Troll King can cast it) etc. My experience is that en Earthquake alone will kill maybe 1/4 or 1/5 of the mages, it is not much, but if your troll survives to cast it one more time, the attrition of his relatively expensive mages will cause Ctis troubles.

A couple or archers well positioned (as close as possible to the front) can be very effective. I've seen like 30 archers kill 6-7 marshmasters before my PD chaff "lost" the battle to the skeletons.