An easy way to get experience points is to get a helicopter, preferably without alarm. Go to any Infected area, and search for water towers containing Hunters. These are easily found by going to the area where the crows are circling. This indicates where an Infected water tower is located. Lock on to it, and blow it up with a rocket. It should only take one rocket with a direct hit. You will get 8,000 experience points per water tower. With 200 rockets in an attack helicopter, you can get 1,600,000 total experience points. Then using the machine gun, you can destroy forty more water towers. You can then use the remaining homing missiles to blow up a hive for more points. Get another helicopter, and repeat the process. The water towers will respawn after a short period of time.

Play the mission where you have to steal a helicopter to find the right Hunter. You are told to go fly over the Infected area and scan the Infected water towers for a suitable enemy type. Instead of doing that, fly to the Infected area, and start shooting the Infected water towers that are around all Infected areas. You get approximately 8,000 experience points for each. Shoot 25 of them to get the "Misconception" achievement for destroying the Infected water towers without them hatching. As long as you properly hijack the helicopter and no one suspects you as an enemy, no one will attack you while you do this. One water tower requires one missile. Make sure to steady it so you do not waste missiles. One flythrough with the helicopter can get you 1,192,000 experience points, assuming you can find one water tower per missile. You can get even more than that if you use your machine gun. After getting about 500,000 experience points, crash the helicopter, and repeat the process again from the checkpoint.

Note: This trick requires the ability to hijack helicopters and have Infected Vision, obtained later in the game. Hijack a helicopter, then activate Infected Vision. Look for the bright orange water towers scattered throughout the city, primarily in the Infected areas. Shoot them with the helicopter's weapons. For each water tower destroyed, you will get 10,000 experience points, which are important this far in the game. Note: Water towers respawn continuously for unlimited experience points.

Use the following trick to gain more experience while running. Carry an army truck or another large object and run through the infected people, or collect the genetic materials that comes out of a hive.