All characters in For Honor parry the same way - by pressing heavy/strong attack towards the opponent that's attacking you. Timing is absolutely key: you want your attack to connect with your opponent's attack mid-air, thus stopping it from coming in and hitting you properly.

Can you parry a heavy attack in for honor?

All characters can parry by pressing Heavy Attack in the same direction the opponent is attacking from. You have to time the attack so that it connects with the opponent's attack before it hits your character.

Why is it so hard to parry in For Honor?

Assassin-classed characters need to be extra careful as they can only guard in a specific direction for a certain amount of time before their guard drains. Once the guard drains, it becomes impossible to block in that direction and thus makes it impossible to parry.