Copper Golem

Lesser Conductivity. Whenever the golem is subjected to Lightning damage, it regains a number of Hit Points equal to half the Lightning damage dealt.

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

Immutable Form. The golem can’t shape-shift.

Short-Circuit. If hit by 3 separate instances of lightning damage before its next turn, or upon coming into contact with water, the golem will use Discharge immediately, and then be Paralyzed until the beginning of its next turn.

Multiattack. The golem makes two Shocking Punch attacks

Shocking Punch. Melee Attack Roll: +2, reach 5 ft. Hit: 6 (2d4 + 2) Bludgeoning damage plus 2 (1d4) Lightning damage

Zap. Ranged Spell Attack Roll: +2, range 20 ft. Hit: 8 (4d4) Lightning damage

Discharge. Constitution Saving Throw: DC 12, 3 creatures the golem chooses (3 random creatures if triggered by Short-Circuit) in a 30-foot emanation, centered on the golem Failure:  12 (6d4) Lightning damage. Success: One-Quarter damage. Only used once per battle, unless triggered automatically by Short-Circuit. If triggered by Short-Circuit, the golem takes half it's remaining hit points in fire damage.