(I did this after completed main game 100% and loaded that save on NEW GAME+ MODE with easy or normal difficulty, so all weapons and skills will be unlocked and easy place to grind. it took 5 minutes to hunt 6 cumulative achievements) For Sixth Sense Achievement, At COZUMEL, after picking up the dagger and you'll come through one enemy near elevator and 3 enemies in beginning of the stealth sequence.. after that you'll jump into a new area where 3 enemies in ground and another one on top of the ruin. So game auto saves here.. with lara hiding on bushes... Now Press & Hold
to Select Perception and kill those 4 enemies with any weapon before perception time runs out. Repeat this until achievement unlocks, achievement will unlock after killing two enemies on 3rd time.
also you can unlock other 5 achievements here...