The Mountain That Walks (Rock/ environment)
Aeons ago, a desperate struggle took place on a faraway mountaintop, a small army fighting to destroy an evil warlock terrorizing the countryside. When the warlock was defeated, his potent magical energies accumulated in the mountain and continued to grow, until one day they gained sentience and the ability to move about,taking humanoid form to do so. Thus was born (Name of boss), the Mountain that Walks.
Moves:
Mountain fog: The Mountain's peak is shrouded in eternal fog, which can be summoned to blind foes and lower their morale. (Lowers accuracy (curse) and defense of enemy units in an area)
Bat Swarm: Millions of bats use the mountain's caves as a roost, the temperature remaining relatively constant within. They will not hesitate to defend their territory if threatened, confusing the invader and overwhelming them by sheer numbers. (Does minor damage and lowers movespeed, use Locust Swarm with lots of tiny riderless Battroll models)
Rockslide: (Deals damage and pushes units back, dealing increasingly greater damage as they are pushed back. Alternatively, use Stampede with boulder models, stunning units hit by the boulders. If the boss' arena is on a slope, make sure the spell is only used where the rocks will be moving in accordance with the laws of gravity.)
Fissure: The great maws of the earth open up, swallowing units whole. When the Mountain is defeated, they may be retrieved. (The spell fires off in a straight line, with the smoking fissure doodads appearing at the bottom, lowering the terrain underneath it, any unit caught within it is trapped within the fissure unless it moves away immediately. And if this is for an RPG-type map, a good way to introduce an underground dungeon crawl / rescue mission...)
Slam, Shockwave, Bash, etc.
The Mountain that Walks is a gargantuan unit, able to strike at both air and ground units, though he is of course vulnerable to both. Use a particularly large Mountain Giant (maybe with an alternate skin), with very high life, armor and attack but horrible move and attack speed (it is a mountain, after all). Clouds surround its head, while occasional flights of bats are seen emerging from it.