Do you know anything about ASM? Personally, if I were to attempt this, I think I would start with the Adventure module, and then turn the win screen into my battle area. You might be able to access monster data from the screen you are on when you enter a battle to set up specific battles. The most tricky part would be writing the script for multiple menus in battle. If you haven't seen the special screens tutorial, check that out. You would probably want some sort of "arrow" object that you could move up and down with input scripts, and a user variable named something like "currentChoice" that gets updated when you select an option or move the arrow. As for the menu itself, I would put my HUD's textbox area at the bottom of the screen (or wherever you want your menu), and load text from there. For example, your transformed win screen could look like this:
--------------------------------------------------------------------------------
| Monster art up here |
| Another monster | Monster number 3 |
| Could have a fourth monster! |
| |
--------------------------------------------------------------------------------
| > Attack! |
| Defend! |
| Magic! |
| Item! |
--------------------------------------------------------------------------------
| HUD with health and magic and stuff |
--------------------------------------------------------------------------------
(This was supposed to be a full box, but it looks like this forum doesn't like repeated spaces. You get the idea, right?)
You could use text entries to load in option text, and just hardcode what each option actually does. I would use a "optionLayer" user var for this. When you hit the A button, the script should read the currentChoice and optionLayer variables, load the corresponding new text entry, and then increment the optionLayer var. When you get to where you want to actually deal damage or something, instead of incrementing the optinonLayer variable and loading a new layer, just subtract from the enemies health with whatever algorithm you want. If you need you could even add another layer after selecting an attack that selects an enemy to attack. Then you could subtract from the player's health for the monster's turn, and show a little textbox for that. You could use the actual in editor HUD to display health and magic. I don't have a good idea for displaying enemy health yet, so good luck with that. Maybe in the HUD you could have a variable that displays the selected enemy's health, you would just need some good ASM. Since you would be using a special screen, you could go crazy with the menu art. Also keep in mind, if you do use this approach, you should probably have opaque tiles under the textbox area, so the player cant see the textbox loading in and out.
When I'm writing this it is very late where I am, so I probably missed some functionality or something, and I'm not smart enough to give you any actual code while I'm this tired. If you need any more specific advice, ask away and I'll try to reply ASAP, just not tonight! Hopefully this gets you started and thinking. This project sounds like a lot of work, but it also seems really fun to make as I'm writing it all out. Good luck to you!
