You get it by succeeding 100% Meter post x300 in Denouement: Final Day | Gate 1 on any difficulty. Succeeding it is landing all Five Just Guard's and doing the counter while having the aggro debuff. So if you want to farm this you'd need a dedicated group of 7 other players willing to speedrun the boss to x300, and everyone else dies post x300 so you can be the only one with aggro. I'd personally wait for a solo Kazeros to be added to farm this. At least in comparison to HM x700 stagger you can farm the achievement multiple times within one run.