The latest introduction of multiplayer has completely broken the parry system because it was too powerful in multiplayer, so it has been balanced to work in multi and it has become useless in solo.
This is how focusing on coop penalises solo players.
Maybe justy by making parry stagger tolerance higher in multiplayer ? Then the parry wouldn’t work with just one player parrying an attack to get the stagger.
Just an idea, I dunno how hard it would be for the dev to implement this multiplicator.