I Did This While In A Party:
- Talk To The Ally, And Click Trade.
- Select Item/Items From Your Ally, Then Trade Items Of Equivalent Worth To Complete The Trade
- The Bar At The Bottom Says "Okay" When You've Traded Enough.
Trading 50 Times Nets You This Achievement.
- You Can Find An Ally With A Healing Drink, And Trade Them A Healing Drink For It.
- Do This And Your Ally Won't Run Out Of Items To Trade, So You Can Trade With Them 50 Times.
Good Luck!