To unlock this you must unlock all operators within any CTU, including two attackers and two defenders. Operators may be unlocked from the Operators section in the main menu. There are five CTUs; SAS, FBI SWAT, GIGN, Spetsnaz, and GSG 9. The amount of Renown required to unlock an operator increases each time you unlock an operator in a given CTU. As an example, if I were to use the FBI SWAT for this achievement, Ash would cost me 500 Renown. Thermite would cost 1000, Castle would cost 1500, and finally Pulse would cost 2000. It costs 5000 Renown in all to unlock each operator within a CTU. Once you unlock your last operator, your achievement will unlock.

If you have not already done so, playing through the single player Situations mode and earning all stars on all missions will earn you around 7000 Renown. Viewing the tutorials from the main menu will earn you 800 Renown. If you have exhausted both of those options and are still short of the required 5000, your best bet is to hop into some Terrorist Hunt and complete as many missions as you can. Depending on how successful you are during missions, you can earn up to about 1700 Renown per hour.

Please feel free to message me if I can make this solution any better. Otherwise, happy hunting!