Caimie's advice is always better then mine, ESPECIALLY in regards to crafting. If all you care about is the gathering job you can level the fastest, how long that would take, and i assume "how do i do that" would be the last question that you left unasked then there actually is an answer. Fishing. I havent personally done this, but would hazard a guess that you could probly do this much leveling on FSH in 5 hours or less. The key is Ocean Fishing. You can unlock the OF instances at level 1 FSH. If your goal was just to get to namazu levels ASAP, unlock the class and the OF system(both from the lower decks in Limsa). Look at the schedule and be on the dock ready to go before the next excursion sets out. Que immediately when the window opens. Ocean Fishing, im told, gives ridiculous exp. Getting to near level 20 on your first voyage is possible. If you are fast enough, you might be able to get a second voyage in before the window closes. After that, you have 90ish mins to do class quests, upgrade gear, and maybe get in a collectable or two(or just fish). What happens in 90ish mins? The next OF vessel goes out. Getting 3-5 runs, possibly with a bit of extra fishing in between, and you should be at the level you seek.

Downside to this method: You level FAR too fast to enjoy actual fishing. You will wind up with a ton of quests on backlog, and will need to finish them eventually to get all your job skills. You will need to gear yourself to actually DO any of the namazu quests(whether thru scrip, gil, or friends). And worst of all, you will have to do the namazu quests as a fisher(maybe the fishing RNG doesnt interact with the quests TOO poorly, but i was never willing to find out).