My favorite way of leveling alts to 50 is through command missions. They cost seals but you will get all the treasure in dungeons that you can trade in to get many of those seals back.

You already get the xp bonus for having a higher level toon, and can take out all the enemies in the dungeon for max xp.

It is slower at first, but as tactics are unlocked and raised, you can pump up your squad's offense significantly.

There is a period before and after Brayflox that I still group with other players due to the wide gap, but apart from that, the xp is pretty nice. Much faster than fates or leves in my experience and is also great for sharpening new skills before you jump in the new class with other people.