How do you get sniper challenges and other challenges?

Use the required weapon by unlocking it with the necessary online rank and choose for it to be your primary weapon (or secondary if you have Overkill as perk 2) then kill the required amount of people or get the required amount of headshots, good luck!

You have to get 100 kills for every sniper, as well as 150 headshots. 100 kills gives you the acog scope. 25 headshots gives you digital camo, 75 gives you blue tiger camo, and 150 gives you red tiger camo. Just keep practising with snipers and you'll get the hang of it. Don't put your controller sensitivity too high and try using the m40A3 sniper (the first one), as it's a good beginner sniper. After you get a kill, don't sit around, move to a new location because the enemy you just killed and other people who saw you on radar will be coming after you. Good Luck!

Uh, you'd have to be specific here. As far as I know, all you need to do is do whatever the challenge says to do the specified number of times, and you should just get the challenge easily.

I think the best way to get headshots is this:
go to find a game on the menu. When this comes up, select "cage match". Cage matches ae 1 on 1 games. If you have a headset, ask the other player if he wants to do headshots. The standard rules are you take turns getting headshots, usually in sets of 3.
If you don't have a mic, hide in the corner of the map, go to "players met" on your XMB. The person you are playing should be at the top of the list. send them a message that says, "headshots?" and wait for a response
And last but not least add me as a friend so i can help you out.
My PSN is: dfrost182

In order to get other sniper challenges and challenges in general, You have to keep ranking in the online multiplayer, the more you rank up the more challenges and weapon challenges you unlock.

These guys pretty much nailed it. Doing headshots is the easiest or if you want to be old fashioned and play regular matches go right ahead.

And if you want to find away to do headshots with friends you trust not to screw you over, add me. My ID is Synyster-Death. If that doesn't work try it with an underscore. I can't remember...

yea i'll help you out to man. add me my ID is Dollartute