After completing the main game, there are 17 targets on the Killboard in the Eva's Hammer main area. Since Frau Engel is automatically marked off upon returning to the hub, you're gonna have to locate and kill 16 of the Ubercommander targets, all of which are in "districts", which are revamped mini-areas that you already visited in the campaign.
The Ubercommanders are tougher commanders who use Kampfpistole grenade launchers and have higher awareness, meaning that you must take them out quickly before they cause a ruckus and alarm the area. They "cost" a certain amount of Enigma codes to unlock their missions, ranging from 2 codes to 8 codes.
Enigma codes can be picked up from commanders during the main campaign, and you can farm for them by re-doing past district missions.
Here's the list of all Ubercommanders listed by their location and the amount of codes needed to access them:
Manhattan, Neu York
1. Klaus Klinger, Nuclear Bunker District, 0 codes (unlocked by default for story purposes)
2. Walter Hauser, Harbor District, 2 codes
3. Heinrich Muller, Subway District, 2 codes
4. Raimund Hofmann, Ruins District, 3 codes
5. Leopold Meissner, Penthouse District, 3 codes
Roswell, New Mexico
6. Jurgen Schlusser, Downtown District, 5 codes
7. Friedrich Baumgartner, Underground District, 4 codes
New Orleans, Louisiana
8. Otto Ackermann, Confinement Wall, 5 codes
9. Victor Kreuger, Ghetto District, 5 codes
10. Daniel Eckstein, Bienville Street District, 4 codes
11. Albert Grosse, Lakeview District, 5 codes
12. Erich Eberhardt, Farmhouse District, 6 codes
13. Ludwig Kassmeyer, Aerostat Habitat District, 5 codes
14. Thomas Wechsler, Oberkommando District, 6 codes
15. Michael Rademaker, Transporthalle District, 7 codes
Riverside Bunker, Neu York
16. G. Herrman, Riverside Bunker District, 8 codes. It should be noted that this district doesn't unlock until you've completed the other 15 Ubercommando missions.
Upon returning to the Eva's Hammer after killing Herrman, you'll unlock the achievement.