Reckless. At the start of its turn, the berserker can gain advantage on all melee weapon attack rolls during that turn, but attack rolls against it have advantage until the start of its next turn.

vital rage 

Whilst the beserker has temp hp it scores an critical hit on a 18-20.

Greataxe. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 9 (1d12 + 5) slashing damage.

tornado of death(recharge 6)

All creatures within 10ft of the beserker must make a dc 15 con save or take 2d12 necrotic damage.The beserker gains temp hp equal to the amount of damage this effect dealt