This is the soonest you can unlock the achievement.
*** Spoiler - click to reveal ***Once you are able to use Remote Links In Gotham Free Roam (complete level four in story mode), you need to use the one in Metro Station Central. This will reveal the location of Sinestro. Mark him on your map, travel to him, defeat him and purchase him for 125,000 studs. Continue story mode until completing level 14. This will unlock Green Lantern. Now, travel back to the Bat Cave, use the computer, and enter any level in Free Play mode, selecting Sinestro as your character. Once the level starts, press Y to switch to your second character and hold Y to open the character menu. Select Green Lantern and beat down your partner, Sinestro, and the achievement will pop!
Alternately, if you wait to get Free Roam Free Play (unlocked after completing story level 15; after the credits roll) before using the remote link, simply switch to Green Lantern, locate Metro Station Central, use the remote link, mark Sinestro on your map, travel to him, defeat him, and the achievement will pop as soon as he surrenders for purchasing.