I swapped my skill bind to CTRL or Shift-RMB
I equipped the Bucker (Thops barrier, carian barrier, the Erdtree one whatever it was called are also fine, I feel more confident in using default Buckler)
You agro the enemy and tank the teleport & grab attack. Do not roll off the floor.
It is important to hold down shift (so you don't backstep by mistake) and wait for the mob to lunge at you.
As soon as he lunges, his weapon will light up and raise his arm. PRESS CTRL / do SHIFT+RMB
If you were successful, you will stagger him, and his HP bar will appear. - hit him till he dies, but be quick about it.
If you weren't, you died. Respawn and retry.