The Orange Box

A great place to do this is in the Our Benefactor chapter. After you get your super charged gravity gun you'll eventually come go up a small lift then go thru a passage on the right (don't forget to recharge your suit in the passage). When you come round the corner, there will be many enemies coming after you in groups of 3 or 4. This is a great spot to get both the Conservationist and Atomizer achievements. The combine ball fields are close enough to throw enemies into it easily.

In the end of Half-Life 2,you will obtain the all-mighty Blue Gravity Gun. Use this weapon to suck in Combine Soldiers, and throw them into the energy ball generators. Do this 15 times, killing 15 different Combine Soldiers, and the achievement will unlock.

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.