I haven't gotten "hardcore" with the title either mtofu. I've gotten mostly Cs on my missions aside from skirmishes. I've gotten a D before and a couple of Bs but mostly Cs for the campaign.
The aces are just around as normal soldiers. The only difference is when you aim at them or focus on them, instead of saying "ShockTrooper" or "Scout" it will say something like "Ty the Immortal" or some other specific name, and they are a bit stronger than average. Killing them earns you a special weapon at the end of the fight. As long as you keep an eye out and are semi-thorough it's not too hard to find and kill them.
You can find me on facebook as Markus Van Rijn, if you friend me just mention you're from VGchartz and who you are here.