A secret that needs some work to get it. Any weapon will do it, but Shredders are far the best, Laser Minigun the second best (you need exactly two Laser Minigun clips). Simply go to the underwater facility at any match, day or night. Then find the trees at the bottom left of the map, and shoot at it. If no leaves are falling down, then quit the game. If you see leaves falling down, then shoot the trees. Shredders are far the best as they constantly "trim" the trees. Even the shredder's description says that it fires flaming sawblades. Some people thought it was actually for katanas, which is wrong.Note - Look at the trees - If they have leaves falling, the go ahead and get the achievement. If there are no leaves, then don't bother.