What can be done against air units in Oklahoma Mission?

[Draedu]

I made it to the Oklahoma Mission on REALISTIC difficulty. But that's where my path ends, as a Legion air unit destroys all my units. It doesn't matter if they are hiding at the edge of the map in a building or at the other end of the map. After a few turns, the Legion unit flies right there and bombs my units into the next life. So far, there is not a single unit that I have been able to buy or found that can do anything against flight units. Has anyone here found a smart solution on how to proceed?

[damart]

ATGM squad have perk to attack aircraft, but I haven't seen one yet.
Also I think .50 HMG and 20mm vulkan should fire air units, try small trailer with vulkan, it seems like it have more gun elevation that Bradleys

[Warnoise]

Use plasma guns. They absolutely shred the HK aerial

[Naoshi]

ATGM squad need to have a perks to be able to switch between anti tank & anti air missile.
At least make sure to have one with

[Draedu]

Thank you for the answers, after more than 20x loading I have seen that almost all second armaments on vehicles shoot against air if they are not direct anti tank weapons. Even my snipers shoot at the air unit. I have two bradleys with getlings as main armament also these shoot against air. Why haven't I noticed this until now, because the air unit fires 6-8 double volleys where just one hit destroys my Abraham main battle tank at a distance where most of my units don't even react. Even if they both shoot Bradley at the air unit, they don't seem to do any damage. In addition, there are two artillery units on the map that destroy two of my units per salvo. Since they can always see units that have been spotted once they have been spotted, they shoot continuously. I have no idea how the game designers came up with this mission.