Sanctuary - "Ashes and Fury" [Campaign 4 - Mission 1]

Reinforcements, by the way, provide an increase of 1-2 creatures in the starting troops. So, the bonus is rather weak. But one of the most useful skills at the start is Tactics 1.
And yes, you also need to level up the water-based control spells – freeze and ice puddle – to damage the enemy while it's slowed down.