Leveling was removed as a focus of the game. It's now something to be done quickly, so you can jump into the events you want to do with a job you want to do. Subjob items are ridiculously easy to get at level 30+, and you will definitely have a Magicked Skull from a Gusgen party.
You can also level solo without Gusgen. I did Sarutabaruta, Tahrongi, Buburimu, and then Battalia getting my SAM to 30 for Abyssea. Took me about 12 hours and then another 6 in Abyssea.
Go do story missions or endgame with friends. Find a linkshell to do them with. 99 is easy to get in a week or so.