Do you want help or not? There is alot of solid advice given to you here.
Like SjMX said this game is getting very old and SG exile is pretty much the only server left wich means everyone on here are veterans with 4000-6000 hours avarage and its all pretty much only exile.
There is alot of small things that noobs probably dont know. Like you said you are getting sprayed by LMG from >1km away. You can do the same but some weapons need a bipod and some don't. Some have long range some don't. Some have good accurary with low damage and some have high damage but bad accuracy.
Players jumping from helis to ambush missions have always been common but probably more so on this server since money is easy to make.
You get spotted easily because you are probably using grass ON and THINK you are hidden when in fact anyone can see you. Or you are wearing bad camo. Besides unless you are hugging a tree or hiding behind one there is no camo that will make you invisible. TURN OFF GRASS.
And that clip you posted of trying to kill the guy jumping around in the forrest, well thats just how it is to fight Jordan.