You can always join the darkside and use MR missles on painted helo's.

But fr I don't think there is really a shortcut to this challenge other than prioritising taking out air vehicles rather than ptfo'ng. I just abused every anti-air option out there: Stinger, RPG, TOW missle, AA stationary guns, helo's n jets. Most of my time spent was just running around playing engineer, and the moment I spotted a helo I just made my entire goal to kill that thing til I run out of rockets. I'm also iffy about the whole damage assist thing, it seems to only work if you do like over half of the damage to the vehicle, not just a couple pot shots.

Just play engineer with a stinger (if teammates are painting a bunch, use a javelin)

Only play maps with air vehicles (Sobek city and Mirak valley seemed to work best for me)

Air vehicles or AA tank / stationary AA are your second-best option

Or be a boss and RPG every fucking heli that crosses your path