This is done during each mission in campaign. Completing all of the tactical challenges will earn you several weapon attachments, as well as the 'Master Tactician & Tactician' achievements (Complete 100% of the tactical challenges & complete 50% of the tactical challenges).
There are a total of 12 missions. Each one has 3 tactical challenges. If you die after completing a challenge you do not have to do it again, it will stay saved after the first completion! You can do the tactical challenges over multiple playthroughs or in Mission Selection. The chosen difficulty does not matter (only for the challenges that are tied to a difficulty - Mission 4, 6, 9, 10).
You do not have to reach the end of the mission to make the tactical challenge count. You can drop out after reaching the next checkpoint. The enemies have the same motion pattern on every difficulty.
Note: You do not have to do the Weapon challenges.
MeonOner Credits: PowerPyx for taking the time to make a very detailed guide with videos located HERE!