The Taking Tree is at the very end of a long winding Mysterious Cave located in the Snowy Mountains which is in the top right of the world map. To get here, transport to the Snowy Mountain Outhouse and look at the area map. You'll see a cave directly to your right, which is the Mysterious Cave. Once you enter, you'll see a Frozen Monk which will give you the quest to kill the Taking Tree. After you fight your way to the top, you'll see the tree and notice that it's heavily defended. It's level 21, so not only powerful itself, but spawns a number of enemies continuously. Definitely make sure you have plenty of health potions here, you'll need them.
The tree itself is somewhat easy to defeat. using the same method as all other bosses, it'll be a piece of cake. Just take your time, and when ever he does spawn out enemies, kill them as they're phasing in, it'll save you a lot of time. You do not have to turn the quest in to get the achievement, but its good for the experience if you need it.