This is the most time consuming achievement in the game. From the way the trials are set up, you must complete a mission normally to unlock the first trial within that mission. You’ll then have to replay that mission, complete that first trial, and then finish the mission to unlock more trials. There are different trials on normal and hard, and you must play on both difficulties to unlock them all. Keep in mind that you must beat a mission on normal to gain access to hard difficulty for that mission.

The "marking" trial requirements can vary. For instance, once you begin the "fastest marking all enemies" trial in the Ground Zeroes main mission a notification will pop up on the left of your screen to track your progress (Marked Enemy [1/40]). While it does say 40 when you begin, certain enemies can leave Camp Omega, dropping the total. Also, when you alert enemies, backup units are called in, raising the amount of people you must tag to complete the trial. Make sure you get high up in those vantage points like the watch tower at the Admin Building, and mark people from there. Circle the entire map. Don't kill anyone prior to marking them.

As long as you play a mission on normal for the first time, go back to it again on normal to mark enemies (if it has that marking trial), play the mission on hard, then replay the mission on hard once more, the achievement should unlock. That's four playthroughs of each mission, two on normal and two on hard ("Intel Operative Rescue" only needs to be played once on normal and once on hard).

You can view trial records in the main menu. Locked trials will have ??? displayed and you won't see their names. Remember, you don't have to obtain a time or statistic in each trial, just make sure that you have no ??? showing up in your trial records.

If you read everything above and you're still confused, this is basically what you need to do. Be sure to complete the mission after you mark all of the enemies on normal. Fastest completion time on hard difficulty is simply setting your own personal best clear time. For "Intel Operative Rescue" all of the trials unlock after clearing it once on normal and once on hard. All of the other missions need to be played twice on normal and twice on hard.

Normal difficulty:

• Ground Zeroes
  • First playthrough (don't worry about trials)
  • Second playthrough (mark all enemies)
• Eliminate the Renegade Threat
  • First playthrough (don't worry about trials)
  • Second playthrough (mark all enemies)
• Intel Operative Rescue
  • First playthrough (don't worry about trials)
• Classified Intel Acquisition
  • First playthrough (don't worry about trials)
  • Second playthrough (mark all enemies)
• Destroy the Anti-Air Emplacements
  • First playthrough (don't worry about trials)
  • Second playthrough (mark all enemies)
• Jamais Vu
  • First playthrough (don't worry about trials)
  • Second playthrough (mark all enemies)

Hard difficulty:

• Ground Zeroes
  • First playthrough (don't worry about trials)
  • Second playthrough (you're going for your personal "fastest completion time")
• Eliminate the Renegade Threat
  • First playthrough (don't worry about trials)
  • Second playthrough (you're going for your personal "fastest completion time")
• Intel Operative Rescue
  • First playthrough (don't worry about trials)
• Classified Intel Acquisition
  • First playthrough (don't worry about trials)
  • Second playthrough (you're going for your personal "fastest completion time")
• Destroy the Anti-Air Emplacements
  • First playthrough (don't worry about trials)
  • Second playthrough (you're going for your personal "fastest completion time")
• Jamais Vu
  • First playthrough (don't worry about trials)
  • Second playthrough (you're going for your personal "fastest completion time")