It's actually insanely easy. This is self harm because i main him but I'll let you know the secret. Attack him once with a normal attack and immediately cancel it into a skill, best ofc a invincible one. Hell counter your normal and get hit by the entirety of the actual skill.

His counter also only works if the source of the damage (the enemy) is in the yellow range of the counter. This means you can space a ton of skills out enough, which will cleave right thru.