Charged Golem

Acidic intolerance. Whenever the golem is subjected to acid damage, it takes double the damage and instead.

Lightning Rod.  Whenever the golem is subjected to lightning damage, it instead heals hit points equal to the damage it would have dealt.

Back-up battery. When the Golem would reach zero hitpoints for the first time, instead set its hitpoints to 40 and the Golem's attacks become random to who they target. Reroll initiative.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.

Electric current. The golem's attacks are additionally lightning. The golem also emits 30ft of dim light.

Multiattack. The golem makes two slam attacks or one slam and discharge.

Arcing Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 12 (1d10 + 6) bludgeoning damage. If the target is a creature and hit, each creature within 15ft of the target must succeed on a DC 15 Constitution saving throw or take half of the damage rolled. 

Electric discharge. Ranged Weapon Attack: +8 to hit, reach 20 ft. sphere, any targets within 20ft. Hit: 10 (1d10 + 4) lightning damage. 

Current Overload. Once per day, after Back-up battery has been activated. Replace actions with:

Multiattack. The golem makes two laser attacks.

Concentrated laser. Ranged Weapon Attack, disadvantage above 40 ft, Charged Golem loses 20 hitpoints: +12 to hit, reach 80 ft. line, any targets in its path. Hit: 30. Reduces target's maximum hitpoints by 10 until the next long rest.