Your initial location:
And here is Astria (cursor is right over him):
For this chapter, bring a thief.
Your first priority is to destroy the thief left of your initial location, as he has the earth orb (broken, though). With Feena’s help, you should reach him in the very first turn.
Now, half of your group should head to the right, where the door will open in a couple of turns. Expect attacks from mages, soldiers, a couple of snipers and worm spells. This is how the enemies are placed, so you can plan ahead where to position your units. Chiki works pretty good in this part.
The other half of your group (your thief should be here) will head towards the treasure room where the heroes are. The best position to assume is this:
This way, only one enemy will attack one of your units, and you can place your spellcasters behind for assisting the attack or heal. Another way to do it is to place your strongest unit (making sure it can take two hits) where Marich is in the above picture, and ranged attackers/healers where I have my thief and sniper. Note to be careful of the heroes with armorslayer and swordkiller. Once you clear the area a little recruit Astria (is safer than battling him).
Finally, block the thief’s escape, kill them and retrieve the remaining treasures, while on the other side of the map you battle the bishops.