Nothing too tough here. Clear out the old Baron Saturday theme park. Use stealth when you can. Takedowns while in cover are very useful, remember to hold
to whistle and attract enemies. Try to pick off enemies from afar as well.
For that, I like the Hartmann .30 (aka the M1 Carbine) because I was frequently getting one-shot kills (on Medium difficulty) with it. It can be found on at least one of the first few enemies you encounter in the park.
Achievement pops after the cutscene as you exit the park.