One very effective way of capping the elite is to take a ranged-only party and bring 3 or 4 pets. Put the party flag just outside the range of the bowman on the walls. The pets will walk all the way in the fortress to get to the bowman and keep everyone busy by dying and getting resurrected at a distance while you pick targets one by one.
It is also possible to pull the Acolyte all the way outside the fortress and out of the range of the groups inside. Only a couple Kournan patrols will gain aggro while doing this.
To make this boss fight easier, consider taking the quest Feeding Frenzy at the same time. Once the boss is down to 20%, Toma will instantly kill it. Note that this is only a good strategy for skill capturing, as Toma's kills do not drop loot.