UPDATE: Ok so reinstalling the game worked. Criteria is you get knocked down twice by the shield bash or charge. I'll update if someone can confirm that or has a different number. Let me know!
The requirements for this achievement are listed zero places on the internet so far as I can find. One guy on xboxachievements said he got knocked back a couple times in his boss guide and it popped. Here's a list of things I've tried:
- Letting him knock me back several times
- Letting him knock me off the cliff (into the abyss presumably as the achievement description would have me believe) with his shield bash or shield charge, he also has a kick but it appears impossible to get knocked hard enough with it to get pushed off the cliff.
- Letting him knock me back with each of his three knockdown attacks: shield charge followed by a shield bash, shield bash on its own, and the kick. These all happen within the first phase of the fight. If there are more than three knockdown attacks perhaps they're necessary. I'll update this if I find anything.