There are 3 dragons that you'll need to free to get this trophy. There's Fafnir, Otr, and Reginn. Fafnir and Reginn are associated with the Brok and Sindri sidequests, so those will help you get the Like Oil and Water trophy.
You'll find Fafnir in the Council of Valkyries, Otr in Veithurgard, and Reginn in Konunsgard, respectively.
To free all of the dragons, you'll need to destroy the 3 runes that keep them in place, then have Atreus stab the mask that's binding the dragon. You will face opposition like Helwalkers and such, but if you've leveled up enough, you should be fine. Just make sure you avoid each dragon's lightning attack.
Here's a video (credit to HarryNinetyFour) for a visual guide on this: