Getting this one before you unlock the MKUltra facility is a bit of a gamble. Choosing new agents to hire that have a lot of unknown traits is probably the best way to do it. During the first chapter of my Mossad run I had to hire one of General Azcarraga's lackeys and he turned out to be a Beholder mole. I didn't test this to see if it was random bad luck or not.
In order to guarantee getting this achievement, wait until you unlock the MKUltra facility in Chapter 2 and interrogate all new agents (new hires, captured Beholder agents, informants, and plot-related agents). With all of them interrogated and an agent working the Comms monitoring system (to help reveal agent traits), eventually you will get an agent with the trait “Beholder Sleeper Agent”. While you can brainwash this out, you won’t get the achievement that way. Instead, take them along on your next tactical mission.
To make that mission easier on you, remove all of the future traitor’s gear and any added compounds in the Body Engineering facility. Mine saw her abilities drop by half and her HP by 25%. This will not prevent the achievement from unlocking, it will just make the agent easier to kill when they do. Alternatively, you could use takedown in combat and turn them into a Saboteur or execute them afterwards. However you want to deal with traitors. The achievement will unlock at the end of the first enemy turn once open combat is initiated. Just cap the first civvy or guard you see with an unsuppressed weapon or enter a restricted area without a disguise.