For this you just need to get each of the A.C.E.S to level 20.
For Marksman you need to get headshots, kill rapelling enemies or kill enemies from a distance, I find the easiest one for Marksman is kill from a distance as you can shoot them anywhere in the body, if they die you get the marksmen points.
Also - For the marksman, you could add that playing Killhouse on Realistic with Low density enemies is good for some rope kills. Go up the stairs to the right of your spawn to the top of the building. Go to the right side and hold the left trigger to get against the crates. Your AI teammates will be able to get most hostiles coming from below and in the building. You can just watch the rope in front of you (face towards the opposite building) and pick off several terrorists that climb the rope.
Thanks go to - Smeds o -
For C.Q.B. The easiest by far to get points in. The easiest way is getting kills in a close distance, just run up to enemies, shotgun in the face and bam, you have the points. This one isn't difficult at all, the other easiest way is killing an enemy from behind which is very self explanatory.
For Assault: If you go onto terrorist hunt, realistic difficulty on Presidio, have gadgets Smoke grenade and C4. Turn to the left and theres a car with 2 crates against the far wall (not the main building) Put C4 down between these crates, go up to wall of the building, crouch against it facing the C4 and throw a smoke grenade down to obscure yourself from view. (turn on thermal vision) Wait about 10 seconds and 7 enemies run onto the C4, you then detonate it getting lots of easy Assault points. This 100% works and is easiest way I found to get assault points...Anyone has any problems getting this to work or getting locations wrong please message me and ill help.
Edit: Also just wanted to point out there are a few other ways to get points in each A.C.E but the ways I have listed are the ways that I found easiest.