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 Surtr falls and after the ensuing cutscene triggers.
Some tips for defeating Surtr: Dodge (
) is your best friend in this fight. Just keep circling Surtr with dodge until he finished his combo (three or four swings) then hit him for three or four of your own, disengaging and re-dodging once you see Surtr begin his next wind-up. At some point Surtr will kneel down -- if the voices scream for you to focus (and/or scream that he's becoming stronger), hit
to focus and use heavy attacks until you break Surtr's power gathering. Repeat above until Surtr drops.