This takes a while no matter where you choose to grind it out. That said, I found "The Battle of Stickopolis
" a rather efficient level for trees.
Right at the spawn there are 11 trees all within a short distance from each other. It takes about a minute to cut down all 11 trees. All you need to do is draw axes and hit
near each tree. Once all trees are cut down, simply restart level and repeat. Also with this level you don't have to wait through a cutscene at the start.