All of these Juggernauts come before you dive off the roof, and arrive in specific helicopters (see Danny's solution above for more information).
As for actually defeating the Juggernauts, you're probably better off playing this one solo. While a team-mate can be a big benefit, with an extra team-mate comes additional enemies and the issue of them having to taze the enemy before you can use StarStreak. StarStreak is, without a doubt, the best way to defeat the Juggernauts in 'Vertigo', and you should take the opportunity to use it to it's full potential.
Be aware that, just like any other Juggernaut you've faced in the past, flashbangs will go a long way to rendering them vulnerable. The best way to deal with them is swap your G36C out for the PKP Pecheneg and keep your StarStreak secondary. As soon as the Juggernaut lands, nail him with the StarStreak, then instantly toss a flashbang. If everything goes well, the Juggernaut should drop to his knees and be open to more assault. Swap to the PKP to nail him in the head and he should drop in no time.
You start the level with 4 flashbangs I believe, so repeat the above tactic on each Juggernaut that turns up, and you should be good to go. Try to be accurate with your StarStreak shots on the other helicopters as you run the risk of running out of missiles. That being said, be liberal with your use of weaponry on any other opponent besides the Juggernaut - do not pull any punches against the Juggernauts as they will easily nail you if you let up before they drop.
The achievement should pop as soon as you kill the three Juggernauts, so completing the level is - as far as I'm aware - not necessary to complete this achievement.