On mission 22/31, where you get the first cell phone cut scene, you'll be asked to jack a Black ops helicopter. Once you have the helicopter, you are prompted to pick up a few Black Ops guys and then destroy a Hive. Do not destroy the Hive!

You now have a helicopter with unlimited missiles, no time limit, and as long as you don't destroy the Hive, you can blow up anything else without it kicking off the alert meter. This also allows you to get out of the helicopter without it ending the mission (useful for collect Memory Strands).

The easiest way to get EP in this is to switch to your Infected Vision and fly around blowing up infected water towers. You can save your game in the middle of this mission if you need a break and do the same thing. You can max out your skills and then some using this exploit.