There are some some useful tricks that you can use to kill enemies and targets without their deaths counting in your game statistics.
The Shoot-in-the-leg Trick
It is possible to shoot an enemy in the limbs (arms, legs and feet) until they collapse to the ground. There life meter goes to zero but they are technically still alive. In this situation it is important not to run them over, or deliver a final gunshot as this will count as a kill against you. This means that for all enemies that are not immune to non-player you can use this trick. It is quite bizarre that on several missions you can use this technique to remove all of your targets.
The Push-object trick
As the name suggests you can use in game objects to crush your enemies to death. This can be done in a number of different ways depending on the situation. An example is that an enemy that takes cover behind an object such as a car will be killed when you drive into the car with another vehicle, provided that there is something solid enough to crush them against.
If your enemy or target is unarmed you can simply shove them until they die. Repeatedly barge into the character (not using the push/attack move). This can be used to knock your target down flights of stairs, into the on-coming traffic and into water.
The method needs little explanation. If you can get your enemy or targets into the water there is a good chance that they will drown and you will not be credited with their deaths in your game statistics.
Call for Police Backup
During most missions you are unable to call the police for assistance using your cell-phone. However, if you can get hold of a police car you can sometimes call for police backup during a mission. In some missions the cops will get involved and take out all your assailants. Note that in most cases the police are utterly useless and it can take a long time to complete missions using this technique.
Call for Dwayne's Back-up
If you have befriended Dwayne after the Holland Play mission, you can call him to send Back-up. This consists of two of his friends who call themselves collectively the New Holland Hustlers. They are most effective from a vehicle rather than on foot (but still very handy). They are invaluable for the mission Undertaker and very useful for the Assassin Missions, Most Wanted, Vigilante and Little Jacob's deliveries (although to a much lesser extent).
The exploding car trick is possibly the most valuable method of killing enemies without gaining the credit in your statistics. It is essentially the same trick from previous GTA games but the game mechanics are slightly different in GTA IV. It is more difficult to damage a car to almost ready to explode, as the car must first set on fire, then burn for a while, side licks will appear before the car finally explodes (you don't have long to get out either). The car will set alight to the ground and other vehicles in the vicinity. This can be advantageous and disadvantageous depending on the mission. The exploding car trick is much less predictable or controllable in GTA IV but is still one of the most effective tools at your disposal.
Speed up the process of setting vehicles on fire by parking in or next to a flames or burning vehicles. Shoot vehicles (in the front grill) with a shotgun a couple of times to prepare them in advance. Then a number of times with a pistol. Shoot motorcycles in the fuel tank area. Boats - always shoot the outboard motors (or that approximate area of the boat).
The 911 Trick
An important additional trick (known as the 911 trick) is essential for some missions. When the vehicle is damaged enough the engine will sometimes stall. You can restart stalled vehicles by making a phone call on the cell phone. It is called the 911 trick but any phone call will start the vehicle. During Little Jacobs delivery missions the default call is to cancel the mision so be very careful during these side-missions.
Death by Falling Against a Rail
If your target or enemy is
positioned on a walkway you can shoot them in the limbs (aim for
their feet or lower leg) to cause them to fall. This can be effective
against all types of enemies including immune to non-player depending on
the mission's circumstances.This sometimes works with walls as well (an example is the second main target (immune to non-player) in the mission Deconstrucion for Beginners. You shoot him in the leg and he falls against the wall and dies - if you are lucky).
Exploding Barrels and Canisters
There are various
exploding canisters and barrels that you can shoot to kill your enemies.
These often count in your statistics but not always. Canisters can be
used to set alight to vehicles, to cause a chain reaction.
Quick Taxi-turn Around (another one of Jasper's top gaming tips)
Using the taxi service for transport around Liberty City can save accidental kills. When you arrive at a mission marker get out of the taxi quickly, then whistle the driver before entering the mission marker, quit the cut-scene immediately. You will appear outside and the taxi should be stopped waiting for you - sometimes a short distance away but still available.