April 24th 2019 - EDIT: 343 keep updating/changing the playlists, which means I have to keep updating this solution to point to the correct playlist to get this in. Originally you could get this in Halo 4 ranked, but Halo 4 doesn't have a ranked playlist anymore so the best way to get this is in social matchmaking now.
As of 15/04/2019, if you want to do this the legit way rather than boost for it, go into the custom games browser under social games, choose the game as Halo 4 (be sure to deselect all the other titles), and select the game type as CTF. This will make every game a Halo 4 game on CTF (so it's actually much easier to get this achievement now than it was before the update that included the custom games browser).
In Halo 4, you get flag medals far easier than you do in Halo 1-3, as there are additional flag medals that don't exist in the older games such as "Flag Defender" and "Flag Assist." This means you can get flag medals just for killing enemies whilst standing near your flag, as opposed to having to actually capture the enemy's flag, or kill a flag carrier.
Once you're in a Halo 4 CTF game, just stand near your flag and kill anyone who comes near it. You will rack up flag defended medals like this, and some flag assists as well when your teammates capture the flag, which all count towards your progress on this achievement. If you get CTF on Monolith, you can beam rifle people from across the map who are nowhere near your flag, yet because you killed them whilst standing near your flag, you'll still get flag defence medals.
This is by far the easiest way to work on this achievement if you're doing this alone and not boosting. I averaged around 18-25 flag medals per game, but if you totally dominate a game, you could get as much as 40 flag medals like this. The higher your skill, the faster this achievement will go. Poor players can expect around 10 flag medals per game doing this, which means even someone bad at the game can expect to get this in around 25 CTF games of Halo 4. Good luck!