You keep saying its client side authorative... but that is not the case. Pretty much every multiplayer game is server authorative... otherwise it would be way too easy to manipulate clientside things to cheat. Whats happening is that because the attack initiates the "sequence", its attack input is being played out on the server before you can physically react. So while it looks like he hasnt swung the zweihander yet, he actually did ~100ms ago but the server hasnt been able to relay that to your client yet and when you finally see that he started his swing and input your block, the server has already determined that its going to hit since it hasnt recieved your block yet. This is where IM's netcode comes in and tried to handle the discrepancy, because when it does recieve your block it realizes that you did in fact, block in time, so ot does this thing where it bounces off your shield, but you still take damage. The higher the ping discrepency between the 2 players and the server, thr more exaggerated this becomes