@Kaarme Isn't it a single target versus mass attack? All the energy would blow away a low level slime and then blast the ground around it, but doing the same against the goblin king, which is probably a high-level monster, and the surrounding goblins would reduce the damage done to something survivable.
Considering she hasn't levelled up much, the amount of MP needed to one-hit kill the goblin king and the hoard would probably be beyond her capabilities.
Then again, this could just be another case of plot-dictates-the-damage, so we should just read the story and have fun.