And it Burns, Burns, Burns Achievement Guide - Hard2Rel0ad
First thing that you wanna do is, buy the detonate upgrade in de menu, now you have the ability to set enemies on fire.
Next find a Bonfire that you can use to set them on fire. Burn 50 Uruks and the achievement is yours!
*Note
We made a video for you guys, so you could see where to aim and how to do it!
Good luck, Happy Hunting!