Ori and the Blind Forest

Spirit Flame is the primary attack that Ori has. The small Spirit that follows you will auto target enemies that are close by and pushing X causes a flame strike to hit that enemy. Simply use it to kill 500 enemies and the achievement unlocks. It's power can be upgraded and the number of targets it hits is also upgradable, enemies will respawn so there is no danger of missing this as long as you continue to kill enemies during your play through.

I was in no rush for this as it is your primary attack and it unlocked before I made it to the second "Dungeon" so just kill everything you see and have fun!