I beat this by pure luck. First, I chose the movement points and built the trenches in the bottom right on the edge of the forest. I put the Crow as on the very bottom right hiding as far back as possible. I chose to get zero units to minimize the enemy and then placed the cavalry and all 3 heroes grouped in front to max their stats and movement. I put the elite and the defenceles sin a line to the right(make sure to put these units and the left as far up as possible). When the battle starts, I use the calvalry charge as efficiently as possible to kill as many enemy units as possible(don't save movement points). After that, I had the infantry charge to also kill as many units as possible and also encircle the troll(again no need to save movement points). With this setup, The king will charge into the encirclement by himself while the rest of the army goes after the crow.
Edit: If this doesn't work, tweek the setup a bit