The easiest way to get a guard's dog tag is by, as already been said, pointing the gun at their back, close enough, so Raiden or Snake will yell "Freeze!" Now, as they are with their hands up like idiots, you got to walk infront of him to point the gun at their face so they'll ask you not to shoot them. The only thing you have to do is to keep L1 pressed at the mean time you got pressed square, so you can walk infront them while still aiming your gun (if you stop aiming they may try to take advantage and shoot you) if you understand all what I've said, you'll see that is freaking easy to do it.
But come on man, that game is freaking easy in normal. Even for newbies. Try it
