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.