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I suggest you try out GameMaker it's a lot easier for what you want to accomplish.
I've been trying to create a MapleStory game in RMXP for about 2 years now, I still got no luck on the mapping, characters on map and battlesystem as you said so let me know if you do please. :D
Well, then, you're shit outta luck, 'cause there is not a decent platform script yet that is also compatible with an on map battle system. Try searching on some Japanese scripting sites, though. Maybe you'll get lucky.