There is a destructible wall immediately after you stealth kill the first enemy in the mission Stalingrad it looks like a brown partially see through wooden section of wall. Simply reload the checkpoint when you break through and you will start back in the stealth kill room rinse and repeat 5 times.
Depending on your settings you may have to double tap your sprint button to activate tactical sprint if you don’t have it set to auto activate on movement.
There is probably an earlier opportunity to get this but this was the earliest level that I thought would have one in after I’d finished the campaign and went back to clean up what I was missing.