Middle Earth: Shadow of Mordor - Burning Vengeance - Perform an Execution on a flaming Berserker. [Bronze / 15G]

Berserkers are the enemies with two Axes. To make them burn you have to push them into a campfire. Then you have a couple of seconds to land an execution finisher (Triangle + Circle on PlayStation / Y + B on Xbox).
Any stronghold works great for this. There is one on the far right side of Udn. Look for a campfire and let an enemy set off the alarm. Now lots of Uruks will come running to your location. Although the enemies are absolutely random, there is almost always a Berserker in the group. Build up a combo streak, then throw him into the fire. You can throw enemies by pressing R1 (PlayStation) / RB (Xbox). Perform the finisher as soon as the Berserker starts burning.

The campfire can only be used once. So if you don't land a finisher quick enough, or if no Berserker spawns, then fast travel back to the nearest Forge Tower to try it again.