You can still grind in the Marth one until the others become available. I can tell you that I, and probably many others, spent way too much time on that one map to lvl grind units when it first got released here in NA. The units there are not that hard, especially once you do it a bunch of times, and because you have allies you can make them weaken some enemies and use one of your magic users to finish them off. A smaller reason to use that map is because there are two shiny tiles that always randomly give any character who stops there: equipment, EXP, or (when paired) relationship bonus. So you can earn easy EXP if your lucky enough. This was of course until the spotpass characters and the EXP DLC showed up, which provided much better opportunities to lvl up your units.
Also, an alternative but expensive method is if your magic units are able to use staffs, buy a bunch of rescue staffs (if available) and use it on someone to gain around 40 EXP everytime without having to battle.