Best place to get stealth kills is on the level breakout, with Taggert.
What you have to do is switch to the knife using (Y), go into the tunnels and crouch down (B)
You will have enemy walking around who have a set pattern. It won't take long to get use their route.
Get up behind one, while crouched and you should see the words "Stealth kill" show up in the middle of the screen. At that point hit (RT) to perform a stealth kill.
You will have done a stealth kill if you stab the enemy in the back and not just a normal slash to the side.
Repeat to as many soldiers as you like in the level, or just restart from checkpoint after a couple.
25 stealth kills later, you will get your 40g
Thanks to JMJimmy & B8TINGU for pointing out it is taggert who uses the knife.