You must parry just as the enemy attack hits. The parry has a short time window of about 1 second. The best method is to hit the parry button as soon as the enemy is starting their attack. Parries work with any equipped weapon or shield (including dual-wielding and small weapons like daggers).

How do you know when to parry in Valhalla?

The only way to tell is to see if there is a symbol above the head, as this means the attack cannot be parried and you should dodge instead. You might even find it's easier just to dodge in these fights, especially when facing quicker bosses who give you little to no time to react.

What level should you be at the end of AC Valhalla?

The recommended power level for this area is 340, which is a massive undertaking. You'll be spending hours upon hours travelling around England to complete as many side-missions as possible, as well as mopping up any leftover wealth that you skipped over during the main story to reach that level.