Personally I found it best to level this in 3TDM with a friend well aware of what you're doing and helping you.
I did Sully's setup with bargain to get T-bolt faster. Since his 3rd booster is for my buddy, everytime my friend died, I was able to see whoever killed him, he also used callout to mark enemies for me and as well guarded my sniper spot.
As for spots, most maps are doable, but I would avoid even trying it in smaller maps like chateau and Desert Village. Just make sure you are a fair distance from enemies. Museum is arguably the best map for it.
My class that I used was:
Long gun: Sully's dragon
Pistol: Your choice
Booster 1: Scoped in of course
Booster 2: Ammo award
The main thing I would try and do is just lurk around the parts of the map that not many players go to, try and choose long range maps like Muesum or Highrise and if you have a chance always go for the headshot.
Also you only need to earn 3 long range kills per match, so try and take it easy and you should have it soon.
Play Lab map 6 and just focus on sniping from one side to the other. You'll be able to get plenty of Long Rangers. It's kinda tedious but also kinda fun because you learn the Dragon way better. How to stay in cover or hit someone else coming out of cover. How to corner shoot with the Dragon (take cover on the wall a little ways away from the edge and then press L1). You'll also start to get a feel for the Dragon's recoil and aim assist (Scoped In makes this easier).
I think the best way is to use a Dragon with reload speed, quickily aim at a target (if they're moving and that's almost always), swing the reticule quickliy towards thge target, and try to time it right. If you don't hit the target with one of the first two shots then release L1 real quick, re-scope the gun, and start the process all over again while reloading when you're clip runs out. This is why Reload Speed is a must for the Sniper. Clip Size is pretty good, too. The aim assist occassionally get sticky and you'll get more markers when it does.
1) Favorite maps for getting TLR: Fort, Train Wreck because you can see end to end from opening spawns.
2) Some maps I wouldn't bother trying to get TLR on like Temple or Chateau although it is possible.
3) If you're having trouble with the Dragon Sniper, remember you need to give it a fraction of a second to settle back before firing again, otherwise the aim will be off.
4) If you haven't watched it already, I show some Long Ranger distances in my TLR video: