Alchemical Acid Bladder Zombie

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

The Alchemical variety of undead are created from alchemy (like Frankenstein sort of).  This gives them higher AC and various other abilities.

The  Alchemical Acid Bladder Zombie is packed with bladders of acid such that each attack has a chance of spewing 2D6 of acid in the direction of the attack.  Piecing attacks have a 25% chance, Slashing attacks have a 50% chance, and Bludgeoning attacks have a 75% chance.  And as a one shot would be boring, the Alchemists spells that created the zombie also regenerate the bladders and the acid such that it may be triggered once per turn.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.