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

Reinforcement, by the way, provides an increase of 1-2 creatures on 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 water-based defensive spells - freezing and ice puddle, to damage the enemy while he's slowed down.

Reinforcement helped me to go through the dungeon without any losses, while clearing all the mobs. Later, with the appearance of Lightning, everything became even easier (enemies often didn't have time to reach me). Also, as I understand it, jellyfish (pearl and coral maidens) attack with air, and air enhances them?
Personally, I played with this setup. Once, I leveled up Blizzard, but I didn't like it. I didn't use Freezing or other spells either.