Pull out your pistol(s) by pressing WEAPON DRAW, then press PUNCH to hit your enemy with the butt of the gun. Your gun will disappear. While he is dizzy, pull out your pistol(s) again and you will automatically grab his head and shoot him in the stomach.
Press PUNCH + WEAPON DRAW as Ghost to grab an enemy and shoot them in the stomach. You can also pick them up then throw and shoot them in the air.
While playing as either Ghost or Niobe, hold FOCUS and go up to someone that has a good sized gun (machine gun or sniper rifle). Press Action. This will make them take the weapon from the enemy and do various things (throw them to the ground and shoot them or smack them with their own weapon).
There are three easy ways to easily kill someone (excluding Bosses, Agents, and Vampires). Disarm someone with a long barreled gun (shotgun, MP5, sniper rifle, etc.). If you grab their gun, and smack them with the butt of it. Try to disarm someone from behind, thereby putting them in a sleeper hold. The best way, though, is to have a gun out and throw (PUNCH + KICK) someone. You will knock them down, then shoot their head.
If you have two pistols, hold FOCUS and jump while in a combat with someone. You will dive forward and start shooting.
Sneak up on someone, and the "Use" icon will appear. When this happens, press USE. Your character should start strangling the enemy. Quickly press FOCUS to swing them to one side. You will then hear the sound of their neck breaking.
When you sneak up on someone and choke them from behind, tap FOCUS and your character will break your opponent's neck.
When you have the chance to get an enemy from behind, press FOCUS then Action to kick the enemy down and break his neck on the ground silently. By doing this, your FOCUS meter will also recharge slightly.
When you need to disarm an enemy that is using weapons such machine guns or a sniper rifle, press FOCUS + Down + Action. If done correctly, you will bounce him over your body, and when on the ground shoot a couple rounds to kill him. This requires some practice. Note: When the move is being done, you can turn off the FOCUS to prevent its discharge.