You will more likely get this after acquiring the Wind Charm (power). On that level you will eventually need the help of the triplets. After they (you) open the gate for the girl Merida, equip the wind charm and be ready. Lots of little rocky guys will come at you. It's a good idea to use the Power Attack as well (B button) if you have charged it enough, for some stronger shots.
There will be other opportunities during the course of the game, specially if you play it carefully and on lower difficulties (I first did it on Normal). You can also upgrade and use Charged shot earth during pretty much the whole game, which gives you minions that chase the enemies and blow up, doing massive damage (usually 2 to 3 shots kill an enemy, no matter which is their natural weakness) from a safe distance.
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