Credit goes to Steam user "THE_LEDGENDARY"
The new(er) version of Office made this one unobtainable, but you can actually download the older version of the map, which had windows that didn't shatter completely once shot.
1) Download the map from this link: https://www.dropbox.com/s/z4pz0bdeua1u363/cs_office.bsp?dl=0
2) Right-click on CSGO in your Steam library and go to Properties. Go to the Local Files tab and then click on Browse Local Files.
3) From the window that appears, open the csgo
folder and then the maps
one. Find the "cs_office.bsp" file, that's the current version of the map. Rename it to "cs_office2020.bsp" or whatever. We will be changing this back to its original name once we're done.
4) Cut-paste the .bsp you downloaded into that same maps
folder. Leave this folder open for now.
5) Launch the game and once in the main menu, open the console and enter the command "map cs_office".
6) Once the map has loaded, select the CT's. Once you spawn in, open the console again and enter the command "mp_warmup_pausetimer 1", which will pause the warmup, giving you plenty of time to get the achievement.
7) With the pistol in hand, walk out of the left exit of that basement you spawn in and fire one round through 14 windows, starting with the row of windows to your left as you exit the basement. Stop at the end of that row of windows, turn around and look over the other side for two more windows. After those are also shot out, turn 90 degrees to the right and move back towards the basement. You'll see a couple more windows to your left, to the right of the small stairs of the enterance of the building. The achievement should pop right after window #14.
8) Quit out of the game first
and once you're back at your desktop, go back to the maps folder and delete "cs_office.bsp". That will leave the renamed "cs_office2020.bsp" in there, so rename it back to just "cs_office.bsp".
P.S.: If you make any mistakes in renaming/deleting the files after you've unlocked the achievement, you can let Steam restore the files into a functioning state by right-clicking on the game again, selecting Properties, then the Local Files tab and finally clicking on "verify integrity of game files".