-Note: You can probably do this trick with any helicopter. 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, however, fly to the Infected area and start shooting the Infected water towers that are around all infected the areas. You get about 8,000 experience points for each. Shoot 25 of them and you will get the achievement for killing 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 this is happening. One water tower requires one missile. Make sure to steady it so you do not waste missiles. One flythrough with this helicopter can net you 1,192,000 experience points, assuming you can find one water tower per missile. You can earn 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 only works after acquiring the ability to hijack helicopters and have Infected Vision, received later in the game. Hijack a helicopter and activate Infected Vision. Look for the bright orange water towers scattered throughout the level, primarily in the Infected areas. Shoot them with the helicopter's weapons. For each water tower you destroy, you will receive 10,000 experience points, which are vital this far in the game. Note: Water towers respawn repeatedly for unlimited experience points.
-To gain more experience while running, carry an army truck or another large object and run through the infected people. You can also collect the genetic materials that comes out of a hive.