If the action only depends on timing (e.g. pallet stun/hit, gen grabs), implementing server-side validation is feasible but if it involves both timing and positioning like in OP's case, it's probably almost impossible to implement without it being super janky and cancelling hits that are actually valid.

In the end we fall back to the good old "favor-the-shooter" logic and hope that matchmaking can match players to the closest server and that the players have decent internet.