Season 2 : Challenge: 2 : 9K38 ILG Launcher

It is doable, but it's a bit frustrating. I got the 5 kills across two back-to-back matches of Escalation on Golmud in the end, but there were a few false starts where I couldn't get any before that. If it helps, I found it slightly better to target planes over helicopters (3/5 of mine were plane kills, and 2/5 were Little Bird kills), and it's probably best not to bother targeting the transport chopper unless it's low health already. Also, for one of the two matches, I hung out next to a mobile AA that a teammate had spawned, and let them bait pilots into using flares before I'd fire. 

It'd be much simpler if forced bailouts counted, and I'm also not convinced that the assists are tracking correctly.

I played goldmut 10 times, 2 times firestorm and i had several hits, but even after 2 missiles hit the helicopter was still flying.

It's only doable when having a recon or a teammate, so you can target paint the vehicle is my guess. 

A complete Nightmare. Took me 2 whole Firestorm matches (CQ) and  abismal K/D ratio. GG devs for the most dumb challange

I did it solo; other than the match that I spent camping next to a mobile AA, as they potentially got a few hits on the target. I was taking down Little Birds and jets with 1 or 2 missiles, depending on how damaged they were already. When I wasn't camping the mobile AA, I fired one missile to get the pilot to use flares, then quickly followed up with two more before they got out of range.