The Better Together trophy / achievement is unlocked after defeating Hrist and Mist in battle, the two Valkyries that you encounter in Muspelheim during The Summoning main quest. You should be well equipped by this point in the story to make the fight easy enough, but you can always come back if you find yourself struggling, or adjust the difficulty settings if you think it’s necessary. Fighting two Valkyrie at the same time may seem intimidating at first, but given that this is a story fight, it’s one of the easier boss encounters in the game.

Better Together

Battle Hrist and Mist