Easy on the first map in Paris (mission special ops in easy), before the end you have a juggernaut easy to knife because you are a juggernaut too ;-). Just 5 min and 15 G...
Edit: thanks to Lightsup55 and Ben Reynolds for theirs precisions.
1) As lightsup55 put, the mission is Toxic Paradise.
2) You need to be level 5 in Spec ops to unlock it.
3) You encounter the juggernaut after the 4th canister, in an alleyway through the restaurant bit.
4) If you retreat back to the restaurant, between the restaurant and the other building there's a wall you can run the juggernaut round while you stab him.