How do you make ace in missions? Either general hints or specifics are good. So far I have made Veteran on the ones i've played but cant hit any better.

just try to finish as fast as you can it should help also do stunts

Being fast is the main way to get an Ace rating. First of all, if you kill targets fast in a row, you get chain bonuses that really add up to your score. Second, if you finish the mission below a certain time threshold, you get a point bonus which is quite significant.

Also, try to do at least some of the stunts when you're near them, to get some easy extra points.

Alternatively, you can forget about speed and make a lot of points the hard way - by destroying everything on the map and doing all the stunts. This, however, doesn't work in the missions where you are fighting against the clock.

So is it the prestige points or what? Whats the threshold

Yes, the rating depends on how many points you make. The threshold varies depending on the mission.

in my case i have to ask how NOT to make ace...

somehow all my missions got ranked like that, for me doing all the stunts was natural

Well, I finally got the one on the very first mission by getting it under time, so the only one I haven't got ace on (that i've done) is "special delivery"