I don't think it's too ambitious. I just want to make something with a character attacking monsters on an overworld.
This is very ambitious, especially for RPG maker, nevermind RPG Maker XP which has a lot of issues and limitations.
Short answer: I recommend another engine if you will be making a game like this. RPG maker is not designed for Action RPGs and does a rather poor job at it. Especially with its a grid-based map system.
Long answer: RPG Maker XP, additionally, is an outdated engine that has performance issues and the like. It's really not newbie-friendly, either. If you do manage to figure out how to do what you want to, you'll be bottlenecked by RMXP's engine performance issues and limitations.
If you are dead-set on making an action RPG in RMXP--which again, I
highly advise against doing--you could try Blizz's ABS system here:
https://forum.chaos-project.com/index.php?topic=106.0
Be prepared to understand how to use custom scripts, create custom sprites, etc. to execute your idea. I recommend planning everything out fully first so you get a grasp on just how big of a scope your "simple" idea has.
Otherwise, you can try an engine like Pixel Game Maker MV or Game Maker Studio which are better designed for games like Zelda and Pokemon.