In the first chapter of the Siege of Dragonspear you will eventually be able to travel to certain areas of the city (Baldur's Gate)
Head to Sorcerous Sundries, make a new save. Speak to the shop keeper Halbazzer behind the counter and agree to help him by collecting a box of potion bottles from upstairs. By holding down the
button you'll see all the containers that can be searched. Once you take them from the bench cabinet 4 enemies will teleport in, simply fight them solo and achievement unlocked. The fight is fairly easy but if you have trouble then lower the difficulty.
If you have already recruited someone then go to your bio page (The head Icon in the wheel) and click on 'Reform Party' to remove them before heading upstairs