Story Related achievement, unmissable.
This achievement will be your second or third unlock, depending on what path you take first (Valravn on the left or Surtr on the right, you have to do both so the choice on which to do first doesn't matter). The achievement unlocks after Valravyn falls and after the ensuing cutscene triggers.
Some tips for defeating Valravyn: The best strategy is to use a mix of Block/Parry (
) and Dodge (
). Almost all of Valrayn's attacked can be parried, with one or two exception (the jump attack, the scream attack), but all can be dodged effectively. Parry however allows for counter damage and counterattacks so its best to parry when given the option.
Here's my recommendationed responses to each of his attacks:
Sickle throw: Valravyn will throw one sickle at you, followed by two sickles. These can be Blocked or Dodged, or if the Block is timed right, the sickles are Parried right back to Valravyn to allow for stun/counter-damaging. Parrying the first attack will cancel the second, so Parry is the way to go over.
Melee attack: If in melee range, Valravyn will swing at you. Parry and counter-attack these.
Charge: If Valravyn is at a distance, he'll bend towards you, then fly right at you, plowing through you. Blocking will get you knocked down, though a successful Parry will know Valravyn down; I however found Parry timing here to be tricky so I usually just used Dodge.
Jump: Valravyn jumps in the air and slams down on where you were standing. If you see Valravyn jump up, Dodge (Block is ineffective here). If you Dodge properly, Valrayn will get stuck in the ground for a few seconds, allowing for heavy counter-attacks.
Bellow: At short range, Valravyn will bellow at you. Dodge sideways.
Gather Shadows: Periodically, Valravyn will summon shadows and cloak himself in them, which I think makes him invincible. If you see shadows on Valravyn, hit
to focus, removing the shadows and giving you a good chunk of free time to wail away on Valravyn. Charge in using
, and hack away.
Summon Norsemen: finally, periodically during the fight, Valravyn will disappear and summon Norsemen adds. These are just like the Norsemen you've fought so far during the game, so keep killing them (and watch out for ones that summon in behind you) until Valravyn returns.