A good training spot for Marauders.

Go to Dual Ghost Pirates. It's an empty map and it's very good experience and mesos compared to the others. You don't get hit with magic attacks unless you jump and they're easy to kill.

This is exactly what I've been doing. It may take a few to kill them, but with the double experience (or 4x, if you are using a card), it's really good. I've been getting about 60% an hour so far.

If you don't like duals, you can always just try the ghost pirates. Though, the Duals map is really good, if you use transform when you train.