To get the trophy that's called "And it Burns, Burns, Burns", you have to burn 50 uruks by using the Detonate ability.
The requirement for this is exactly that - the Detonate ability, so make sure that's unlocked first.
Once that's done, you can find either a single barrel or a stack of barrels with Uruks nearby. You can find them in the strongholds in both maps, which is where I grinded. All you have to do is aim your bow and arrow at a barrel or a stack of them and fire an arrow at it. If there are uruks nearby, they will burn from the explosion.
Bonfire also help too in this regard. If you see a small group of Uruks camping around near a bonfire, make that fire explode and it will also hurt or kill the Uruks through burning as well.
Happy hunting and good luck!