Off balance shots are a lot more likely to happen if you're using a player with a low 'balance' rating. I got this with my created virtual pro in a player career mode, controlling only my pro (not the whole team). The first goal I scored, my player stumbled a bit after shooting and then the achievement unlocked when it hit the back of the net.
This achievement will likely come naturally when playing the game, so I would not concern yourself with this achievement.
If you want this achievement quickly, create a striker with really good stats, most importantly a high finishing statistic. Then put his balance down to below 50 and stick him on any team you like. Play some matches with this team with this player up front, and you will likely score an off balance shot very quickly. If you want to make it even easier, change up the sliders in your favour, and then you'll be more likely to score when you are off balance.
This video shows a player doing an off balance shot in a game. Kudos to: originalxtex