Time to call for reinforcements. Vahan wants to use a special signal to summon more Dwarves who will help in the fight against the Undead.
Creating fire to call for more Dwarven units is your smallest problem here. You are attacked by the Greenskins. Their Warriors and Shamans are the biggest threat. Eliminate the latter first, even at a cost of your soldiers' lives.
You start near one of the campfires on top of the hill in the south-east part of the map. This is a great spot to defend yourself (1). You can control all the nearby units. Place ranged units on the edge of the cliff (2), don't enter the forest (3). This spot will allow you to defeat the enemies. Thurvid is your key to success. Order him to attack units moving on the road. Enemies who were slowed down by his ability will hinder other units.
Enemy units are within your Riflemen's range. This unit has the same reach as Axe throwers (5 tiles), and they deal a lot of damage. What is more, they have 100% armor penetration which means that they always deal full damage. Use their reach and firepower to eliminate Shamans as fast as possible - these enemy units deal a lot of ranged damage and they can strengthen their allies.