This is a Metal Gear Solid 2 achievement.
This is the hardest and most time consuming achievement in the game. To do this, you need to collect dog tags by holding soldiers up (point a gun to their backs when really close). When they’re held up, go in front of them and point the gun to them while in their sight. Most soldiers will then start shaking and drop a Dog Tag. Some will play tough guy and refuse to shake, so you got to shoot their hand or feet to intimidate them before they will drop their dog tags. You can also point heavy guns like Rocket Launchers for instant intimidation. Each soldier will drop one dog tag, and every soldier has an unique dog tag. Backup soldiers that come when you do things that are suspect don't count. You must not kill soldiers before you get their tags, and some of them are missable.
Different difficulties also have different dog tags. This means you must hunt Dog Tags from soldiers in pretty much every difficulty level (European Extreme doesn't count as a different difficulty for Dog Tag purposes, it's the same as Extreme) to get this achievement. Also note that different difficulties have different number of soldiers, so there are different numbers of dog tags to get as well. Try to focus on locating soldiers and always holding them up when you can. There are 152 Dog Tags in the Tanker Chapter and 242 in the Plant Chapter, for a total of 394 Dog Tags among all difficulties.
However, there's a trick to how this achievement works. You don't really have to collect ALL dog tags like the description says. You only have to get every reward item. Getting 76 tags on the Tanker chapter and 218 on the Plant chapter will do the job. You still need to finish the game when you reach the minimum amount of tags to receive the item, though, because the achievement is only awarded at the end of the game.
Refer to this thread for further assistance.