You must complete the entire story (Instincts Predator) on all difficulty levels (Easy, Hunter & Predator). This achievement can be a little buggy so this is more of a warning to you. The game will crash and freeze your entire system if you die in certain places. Most noteworthy, the last level! I played through the entire game on Predator 3 times because of different bugs at different points. I highly recommend keeping a backup or saves/save points in case your system locks up and you need to restore a good save. This saved me a few times. When I completed my final play-through (Predator) it only unlocked the achievement for that difficulty and did not unlock this one for all difficulties completed. I can not emphasize enough...DO NOT REMOVE YOUR SAVES! The final checkpoint is after the final boss fight and before the last cut-scene. You can load up and re-play on each difficulty from the last save and it should unlock this achievement (assuming it did not unlock for you when it should have). Good luck!