To unlock an operator, tab over to the Operators section from the main menu and then select the “Unlock New Operator” option. Each of the five CTUs have two attackers. The acceptable operators for this achievement are Sledge, Thatcher, Ash, Thermite, Twitch, Montagne, Glaz, Fuze, Blitz, and IQ. This will cost you 500-1500 Renown depending on how many defenders you have unlocked within the CTU. Once you confirm your unlock, the achievement will pop.
Please feel free to message me if I can make this solution any better. Otherwise, happy hunting!