You should probably start with being more specific. What rank are you? I'd say the best way is to find a group you can consistently leve-link with and just try to leve at camps 5 ranks above you.

So, do leves/behest at the R1, R10, and R20 camps until you hit R25. Then start doing Nophica Well's and Nanawa Mines leves with friends until you hit R33-R35.

At that point try to switch you're party to doing the infamous "Static Leves" at Camp Broken Water. You can pretty much find yourself doing these same leves from 35-50. Just raise the star difficulty as your party goes up in rank, and try to aim at having the mobs be about 10 ranks above yours. You'll get to 50 in no time.

Static leves, as I know them, are (Camp Broken Water): Dunesfolk for Dinner, Ravine Reveries, and All Cracked up. (Nophica Wells): Necro- Levinshower.

I believe there is also one in muy-tun cellars that some people use for static, but I can't remember its name.

I hope this helps. While it may be one of the faster ways to rank up, its certainly one of the most repetitive/tedious. Good luck!

-Aera