I just unlocked this achievement and I am updating this solution.
The earliest you can earn this is the middle of chapter 13 of Half-Life 2, "Our Benefactors".
After you pick up the upgraded gravity gun, practice the upgrades on about 5 combine soldiers. Follow a linear path along a walkway with a low fence. Many more combine will attack but kite them close to the second Dr. Breen monitor. Just past this monitor to the left as you face the monitor, is a short hallway with the first stasis field. It looks like a tall column of white light.
If you did not dissolve the combine soldiers, then you will have bodies available to throw into the energy field. Throw them all from the fence because when you jump over the fence a number of combine will run into the room with the stasis field and attack you. Use the left trigger to suck them up and right trigger to toss them into the energy field until the achievement pops. Stay here near the energy field until you reach the number 15 and the achievement will unlock. The soldiers may stop spawning so keep throwing the bodies and make sure they touch the white light and get electrocuted. The in-game counter will keep you posted as to your count. You can throw in "live" soldiers well.
In this game it is best to get as many achievements as possible during a current play-through as there is no chapter select after you finish. You can reload saves but they may not be in the desired area.