Kill 90 cannibals.

This achievement can be unlocked during the Yamantau level. After getting free of the cannibals, you will be tasked with finding and rescuing Anna and killing dozens of cannibals in the process. You will kill the majority of the 90 cannibals while rescuing Anna but after you rescue her and are in the Comm. room with the rest of the team, make a quick save just to be safe, then as you get back on the trolley to exit the level, a massive mob of cannibals will be chasing after you. Here is where you will need to get the final kills you need to get to 90, so have grenades and molotovs on hand and as you're leaving on the trolley, throw all that you have into the chasing mob. If you don't get the achievement here, reload your save and try again, or else you will need to start the entire level again.

Don't skip any enemies on your way to Anna either. You have as much time as you want to complete the level so be methodical and kill everyone you see.