Start the Season of Revelations campaign as either the Wood Elves or Argwylon.
If you are the Wood Elves, you can get this really quickly. After upgrading a settlement to level two, you'll get a group of Wildwood Rangers in your garrison and the achievement will unlock.
The other way to get this is to build a Selathoi Gallery and upgrade it two times, to a Forest Lookout. You can then recruit Wildwood Rangers.