I really think that the best way to solve the "grindfest" issue is to make the training easier as the levels increase (aka, your duck is getting better at the task). Still give experience on the same scale, and then you'll level quicker as you get higher levels. That, and give some more varied challenges at farther distances, etc.
I think that it should have been that if you destroy a wall that's not on a path, the wall crumbles and leaves behind a pseudo path that can then in turn be bombed and destroyed.
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