This is the award for rescuing 300 captive souls to earn this just kill as many grubs as you can while playing the game. Each unique grub you kill will free one soul (you can either cut the balloon or juggle them til it flies away).
In addition reapers will release souls as well. There's over 550 souls available in the game, and you'll be killing grubs in every level. If you don't have this by the end, replay levels that don't have all the souls saved (you can't earn a new soul saved unless it's the first time you killed a specific grub)