He's easy to keep on the grate if your tanks pay attention to Pawbuster's movement.
Sidestep as Pawbuster backs up and gets ready to unload his massive overhead swing. Then after it lands, get back where you were. Pawbuster doesn't move because he's rooted and stunned the entire time.
The knockback is relevant to where his axe lands. If you're inside the radius of effect (which circulates from where the axe lands, not from pawbuster), you are knocked back. If you're not...you're not. The radius is marked on the ground when the hammer lands, so watch it a few times and you'll get a feel for it.
It makes me sad how few people want to do this encounter like it really should be done - a long console boss fight
