Lixius uses the nearby cemetery to increase his army of Undead.
Your goal is to eliminate all the enemies. The hostile army consists of a wide array of various Undead units. The enemy is strong, has a large army, and has its units placed in hard to reach spots.
Two narrow paths lead to the cemetery (1). Attacking the enemy positions will cost you a lot of units. It is much better to wait for them to come to you. The biggest advantage you can get is to block at least one passage, preferably the southern one. If you manage to stop an enemy unit with Karric's attack, you will force the Undead to use the northern entrance or wander the cemetery aimlessly. These passages have chests with a Lightning scroll. Keep your units close to the starting location (2) - these are good spots to defend yourself.
Mages are your priority because they can deal a lot of damage and have great reach. Use Mortars, Riflemen, and Axe throwers to eliminate them. If you have to, summon more of these units. Your army has a lot of infantry units so there is no point in summoning more melee units. You also have Boars of War. The map's construction allows you to use powerful charges on units leaving the cemetery (3). Wait for the enemies patiently. Don't try to capture the cemetery - this is both difficult to do and not very cost-effective.