As you open your map on pipboy you'll notice 'black & white' lines going horizontal & vertical along the edges.
Go 4 white lines from the left & down 5 white lines & you'll find 'Jalbert brothers disposal' clear out the enemies & within the warehouse you'll find a desk with a terminal on it & to the left a safe on the floor.
The safe has an expert lock, the terminal is set to advanced.
Pick the safe then hack the terminal, within the terminal you can relock the safe & then pick it again, repeat untill achievement pops.
There is also another location just southwest called 'arcjet systems' that has two more set ups like this, one has an expert terminal & an advanced safe & the other also has an expert terminal but I think it was a novice safe if I remember right. However above the safe is a cabinet with the password for the terminal so this may be the best option for people with a low Bobby pin count or a low lock pick skill.
However, to reach this one