Encounter: Bridge (Chapter 8)
The bridge encounter in chapter 8 is perfect for this (you can select it from the main menu). Doing a vertical stealth takedown (drop onto enemy below you) automatically counts as a "normal" stealth takedown at the same time. So when you get the "I Thought I Heard Something" trophy you will automatically unlock the "Shh Sleep Now" trophy with it.
The very first enemy of this encounter is perfect. After a few seconds he will walk towards you. Then you can drop onto him for an instant knockout. Restart the checkpoint and repeat 30 times.
Be aware that if you quit the encounter you'll have to do 30 takedowns again! You must use checkpoint restart 30 times. Your kill counter resets every time you load up an encounter.