The Igla, and it's challenge, are both terrible.
The Igla has to be the worst fucking thing I've ever had the displeasure of being exposed to. Whoever decided to lock 3 weapons/a gadget behind a challenge to down 5 aircraft with the Igla is a sadistic asshole. It's already laughably easy for aircraft to avoid SAM missiles in this game, but the Igla losing lock constantly just makes it infinitely worse.
Is it a day of the week ending in "Y"? Lock lost.
Is there a single pixel of a floating leaf in between you and your target? Lock lost.
Is there a flare from another aircraft anywhere in a five mile radius of your target? Lock lost.
Did you breath slightly odd while tracking a target? Believe it or not, Lock lost, go fuck yourself.
Edit: Got it done, thank you to the people who suggested Hardcore, that was the way to go.