Rage. At the start of imitative, the zombie berserker rages. Raging grants the zombie berserker resistance to bludgeoning, piercing and slashing. 

Reckless. When the berserker makes a melee weapon attack on its turn, it gains advantage on all melee weapon attack rolls during that turn, but attack rolls against it have advantage until the start of its next turn.

Undead Fortitude. If damage reduces the zombie to  hit points it must make a CON save with a DC of  + the damage taken, unless the damage is radiant or from a critical hit. It has advantage on this saving throw. On a success, the zombie drops to 1 hit point instead. 

Multiattack. The zombie berserker makes three greataxe attacks.

Greataxe. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 11 (1d12 + 5) slashing damage. 

Great Cleave. (Recharge 4-6) The zombie make a greataxe attack against each creature of its choice within reach, and then it can make another greataxe attack against one target of its choice. The attacks gain a +2 bonus to damage rolls.