Strategies for all Difficulties
An indoor tight space map. The catch is enemy reinforcements will arrive from the stairways outside of your range, on the left and right sides of the map. They will have ranged attacks to hamper your units throughout the level, and there will be reinforcements in the main area as well.
There are chests on the map, so be sure to bring someone to open them. You will want to stay close together and have healers ready when the enemies start attacking from far away. You may have some spells of your own that can hit them, but more often than not you will not have such.
The Sages all over the map have long ranged magic as well, and will be raining magic down upon your units as you proceed. Take them out as quick as you can.
To win the level you must defeat the boss, so it is possible to rush him before being overwhelmed by the reinforcements.