Pretty much what everyone else said. Solo Leves, and when you can get them Grand Company Leves. If you have a promotion to top rank you get extra leves that are NOT 15 min each...and one that is 15 min but is "kill as much as you can!". Good EXP, good seals.

Mid level like 30ish if you get a pty you can go about the Beastman encampment, keep away from the high level spawns and grab the weaker ones.

40's Beastman Strongholds are awesome for exp with a GOOD pty.