Undead Fortitude. If damage reduces the zombie giant to 0 hit points, it must make a Constitution saving throw with a DC of 3 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead

Multiattack. The giant makes two greatclub attacks.

Greatclub. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 18 (3d8 + 5) bludgeoning damage.

Rock. Ranged Weapon Attack: +8 to hit, range 60/240 ft., one target. Hit: 21 (3d10 + 5) bludgeoning damage.

Upon reaching half health, the Zombie Giant can Stomp in anger as a reaction. 

Stomp. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage. The force of the stomp causes any creature within a 10ft radius of the target to make a DC 15 Dexterity Saving throw, or be thrown back 10ft and land prone.