Unlockable within FOREST GROVE
Simply complete the level only using your bow. I would suggest just replaying the level to get this one after beating it normally, because you will miss out on a good number of coins if you decide not to use your sword first.
You need coins to buy upgrades, such as the charged and power shots, which you need for kill achievements. Whenever you see plants, you can destroy them to get coins, but you can only do that with your sword.
This is the easiest level to do it on, so just beat it once normally, then go replay it and just use the bow.