The Ent Draught is one of the three treasures you'll find on the ninth story chapter, 'Track Hobbits'. You'll find it after you begin playing as Treebeard, located on a tree you plant.
Once you have the Ent Draught, enter any level in free play and press and hold and select the fifth circle going clockwise. From here you can select ANY character, so select Pippin. Switch characters with , and press and hold and select the fifth circle going clockwise. Find the Ent Draught among your many treasures, equip it, and hit Pippin with .
If you wish to unlock this achievement in Middle Earth rather than in a story chapter in free play, you will need to wait until you've completed all of the story chapters, since free play selection of characters is not available in Middle Earth until then. Pippin may also be available for you depending on your progress in the story.