This is not a guide but rather a tip.
This trophy tends to glitch for some players (for me as well, never easy trophy hunting I guess, ha?) and they don't get it after defeating their 10th High Dragon. If that happened to you as well don't worry. Just load the game before the fight and slay it again.
If you are doing you first playthrough on Nightmare like me then don't leave last dragon in "Emprise du Lion". I ended up defeating twice Highland Ravager. Even if Highland Ravager ended up beign your 10th dragon to defeat it is still easily doable even on Nightmare with Rogue, Blackwall, Solas, Vivienne.
If you have more questions I am happy to answer them.