The achievement unlocks upon successfully picking the 50th lock. It is not challenging. There are quite a few hundred locks in the game. You get credit for picking the lock whether you force it by pressing
for a random chance to unlock or whether you use a pick. All locks that can be picked or forced open count towards the achievement. Chests are the most common source of these locks but gates, doors, cabinets, and the like may also have eligible locks.
The percentage probability for a forced open increases as your lockpicking skill increases. However, even very hard locks can be opened with a lock pick at skill level 1. Accordingly, it is better to pick locks rather than forcing them open. If you are picking rather than forcing, increasing lock picking skill is low priority. This achievement can be unlocked (no pun intended) with minimal difficulty at lockpicking skill level 1 provided you carry plenty of lockpicks. Lockpicks can be bought inexpensively from a variety of merchants. The Corner Shop in Gorhart, likely the first (or one of the first) merchants you visit, sells 16 and they replenish every couple of days (game play time).
It helps to have a system. I start with the middle of the left side. If the lock breaks immediately, I go to the middle of the right side. If the pick breaks immediately again, I try middle of the bottom. If the pick vibrates, I know I am close so I adjust in one direction or another. The mechanism shown on the game screen looks complex, but the solution is simple--guess the position of the lock pick with
and move the slide left to right with
. Carry plenty of picks so, if you fail, try again with another pick. If you barely nudge the slide with
, you are less likely to break the pick & can back out without losing the pick. Move the pick to a different position and try again.
Lockpicking is easier than the pictured mechanism in the game would suggest. It is simply a matter of guessing the correct placement of the pick using
. When you pick your fiftieth lock, the achievement pops as shown in the very short video below. Note the unsuccessful attempt on the left. The pick is repositioned on the right to unlock both the lock and the achievement: