Easy achievement. When you get to the Forest world you will build some buildings that require a person for it to operate such as the lumber mill, wood carver, machinist, sculptor and so forth. What you want to do is build the building up to the point where it is completely built but still grey(meaning you need to add a person for it to be complete). Then in the world there are already some cannons built to the left side of the map, so you want to go grab one of them (I took one of the smallest ones) and place it as close to the building as possible. Make sure it is aiming at the building and from there you just need to pick up a kefling and place it in the cannon and achievement unlocked!
Note: When picking a Kefling, make sure it is one without a job already otherwise you will need to remove its hat first before sending it into the building. (QuickMythril's reminder)