Unfortunately, there currently isn't a Capture the Flag playlist to easily get this achievement.
That being said, it's still possible to earn by playing in the "Team Arena" or "Solo/Duo Queue Team Arena" Ranked Arena Playlists. However, the playlists mentioned contain a mix of Slayer, Capture the Flag, Strongholds, and Oddball. So 75% of the time you're unfortunately not going to have the game type needed. It's not the greatest option in the world, but it's there.
One thing to note is, from my personal experience, in Team Arena is that Slayer and Capture the Flag seem to be the most common of the four. However, your mileage may vary on that.