The Light Burst is a skill you receive once you find the Skill Tree in Lost Grove. This allows you to attack enemies and to light specific torches throughout the map.
You hold the
and that will show you the trajectory that you will throw the light grenade. You can adjust the arc with the
. Once you have the trajectory you want, let go of the
and Ori will throw the grenade.
The cheevo requires you to kill 50 enemies. The enemies are the weakest in the ealier map locations, Sunken Glades and Hollow Grove. Just keep running back and forth lobbing the light grenades at anything that moves.