If you like to use melee a lot, you could get this one naturally. Combo takedowns are cool, when you and Nadine are close to eachother and you beat up an enemy, instead of the final punch being normal and killing the enemy, Nadine will jump in and do some fancy ass WWE finisher with you. There are loads of different locations to do this, but it works best when Nadine has no gun. Stealth missions are best since she doesn't shoot as much and will opt for silent takedowns whenever possible.
Perform 10 combo partner takedowns