DayZ

This was pretty buggy when I got it. Infected soldiers are, you guessed it, zombie soldiers in uniform. They can be found in and around military bases/camps. It took around 7 or 8 kills to get this to trigger for me.

Theres an app on mobile devices called "iZurvive - Map for DayZ" which you can use to find these camps/bases. Be careful though, as other players frequently farm these spots for assault rifles and other gear.

DayZ has been patched so you can't get achievements on community (private) servers anymore.

Doing this achievement legit was somewhat frustrating for me as the soldier zombies were elusive. I visited a few military barracks sites only to find nobody or normal zombies. Another problem is these sites tend to be unsafe since experienced players tend to hang out there for good gear and will kill you.

Coincidentally I died and respawned somewhat close to a small camp that had 4-6 soldier zombies in it.

On the Izurvive map (https://dayz.ginfo.gg/) this camp is located at coordinates 5793, 3074. It is at a 3 way intersection north of Vysotovo and south west of Novoselki.

Another place that always had soldier zombies in it is slightly northwest of Krasno airfield at coordinates10940, 13400. Be advised if you hear wolf howls, run and jump on top of something or into a building and close the door.

NOTE:
Soldier zombies tend to be a little tougher and inflict bleed more than normal zombies. I recommend finding a nearby truck or car to jump onto. This will save your life if you aggro several zombies at once. You can typically beat a crowd of zombies to death in relative safety standing on a car's hood.

If you found soldier zombies elsewhere, let me know and I'll update this solution to include the location.