This achievement requires you to use the DETONATE ability to kill 50 Uruks in total. The detonate ability is the 3rd from the right on the first row of the upgrade abilities page.
Once you have purchased this, you can explode camp fires or barrels by shooting at them (
HOLD to aim and
PRESS to fire). Simply find groups of Uruks hanging around either explosive object then stand back, take aim and fire. When you have killed a total of 50 the achievement will unlock.
Note; You can kill the 50 at different times; they do not all have to die from one single blast.