Tier 1 Achievement in Medal of Honor: Warfighter
Completed the campaign on Tier 1 difficulty
How to unlock Tier 1
First off, if going for all achievements in the game, you'll want to complete Hardcore mode first, using the USB memory stick trick. After that, all you'll have to do is reload the last mission on Tier 1 difficulty and complete it to unlock the achievement. Tier 1 is like hard but there's no HUD.