To get this achievement you will first need to find the Ent Draught from the mission Track hobbit you will require a character that can dig in order to get it.
Once you have it go to the over world and have one character as pippen and the other is personal choice once this is done select the Draught item on the character you chose
and hit pippin with it.
Video attached for a visual guide of finding the item by HappyThumbsGaming.