Clockwork Goliath

Immutable Form: The Clockwork Goliath is immune to any effects that would alter its form

Magic Resistance: the Clockwork Goliath has advantage on saving throws against spells and other magical effects

Magic Weapons: The Clockwork Goliath's weapon attacks are magical

Lightning Overload: Whenever the Clockwork Goliath is subjected to lightning damage, it takes no damage and instead regains a number of hit points equal to the lightning damage dealt. In addition, it regains the use of its Charged Impact action.

Multiattack: The Clockwork Goliath makes two Slam attacks

Slam: Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 3d8+4 bludgeoning damage. 

Charged Impact (recharges 5-6): Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 4d8+4 bludgeoning damage plus 4d8 lightning damage. On hit, the target must make a DC 15 strength saving throw or be pushed back 15 ft and be knocked prone