This achievement is stackable with
so start straight on Realistic difficulty if you want to.
Hunter game mode is fun, you need to eliminate the required amount of enemies in each area of the map before being able to advance to the next part, not too difficult and can be done in co-op.
Try not to get spotted, if your are seen and give the enemy a chance to call for backup. The number of enemies almost doubles, this can be annoying because sometimes the backup guys can spawn behind you. Not literally but if you are almost done with an area and you let the last guy spot you, you will need to backtrack a little bit to go and clean up the other guys that have spawned.
Also you dont physically need to be spotted for the enemies to sound the alarm. I've had plenty of situations where after i have took a guys head off quiet close to another enemy, they become suspicious and overly paranoid (rightly so lol). But sometimes they will just look around and not do much, on other ocassions they have just started to randomly shoot at my last known position despite the fact they didnt see me to begin with. Any explosives used like Frag Grenades, Remote Mines and/or Sticky Cameras will automatically set off the backup to charge in.
Weapons, Ammo and Gadgets can be refilled or changed at almost all of the checkpoints before the start of each section so don't worry about using your gadgets and stuff if you need to. If you die you will start from your last checkpoint with the enemies set to their default number.
Here are some videos that can help you out if you are stuck.
Credit to FluffyQuack for the St Petersburg videos.
Credit to asxdefter for the Russian Embassy videos.
Credit to skorpion47 for the Yastreb Complex videos.
Credit to CompassProductions for the Mozdok Grounds videos.
Credit to Lancastersilk for the Lumber Mill videos.
Credit to samadriel for the Printing Press videos.