Shadow vestal has shield. so archers may not be very effective vs them. IMO your best bet is gathering your mages and using elemental evocation to destory them as quickly as possible. Essentially Your normal troop can't do any damage so buy chaff instead of elite units to hold the line. MA Ermor is somewhat imbalance vs many nations. Even if you can deal with shadow vessels, you may still be swarmed by their free undead horsemen if you can't get AoE spells quickly.
