Let’s come up with a monster.

UNDERGROUND SERPENT -> EARTH KING
Earth King - 150 hp, ability: attacks from underground, the enemy does not retaliate,
at the location where the attack was performed, an obstacle appears (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 of the number of Earth Kings)
vulnerable to air magic, but immune to earth magic.