The treant is an enemy that first appears as a "boss" in Questor rank and can be particularly difficult to defeat that early on. If you have a hard time getting to the treant, the only thing you can do is grind resources and upgrade your character. But to kill the treant easy, do as follows:
1) Play and upgrade until you can start a Questor rank run and get to the treant without fail.
2) Run Questor or lower rank runs and gather a full inventory of physical attack items (axes and spears)
3) Start a questor rank run and save your items. Once you get to the treant, use all of your items in rapid succession to decimate the boss.
4) Enjoy your achievement.