15-30 is slowest part of leveling. Leves are generally not worth it in my opinion, so just find adequate mobs to solo/party. Widdle yourself to level 30 somehow (solo/leves/parties/PL) and it gets better. After that you can start spamming Cedarwood or Humblehearth leves. Both of those camps are really close to town. So set HP in LL or Grid, depending, get leves, run to camp, do leves, return, repeat. You can do that until 40, 42 even.

After that you can do the 40+ parties or continue doing leves. Usually I run with HP in uldah, get Halatali and Broken Water leves, and just chocobo between the camps and return for more leves. I like doing leves 'cause I only need to focus for 2-5 mins per leve and get 8kish exp when in parties you have to be on top of everything all the time. ^^