Assault rifle and light machine gun long-range kill challenges are too difficult

I agree. I've used the Flatline since the Titanfall days. I had to deal with kill enemies with semi-auto, down an enemy right after being revived etc. That's the only way to get it since the Anvil Receiver was removed. I can't  get the long range either. Just an FYI, don't swear because they could lock your thread which is a good topic but also so nothing affects your account. Best of luck dude.