Let’s come up with a monster.

UNDERGROUND SERPENT -> EARTH KING

underground serpent - 130hp, attacks from underground, the enemy cannot retaliate.
earth king - 150hp, attacks from underground, the enemy cannot retaliate,
an obstacle appears at the location where the attack was made (a PIT, the unit will not be able to cross it, unless it has the ability to fly).
5% chance to eat the opponent (number of opponents = 1/2 the number of earth kings).

vulnerable to air magic, but immune to earth magic.