Well, since you asked a fairly straightforward question and you didn't use all caps, I'll try to help. (Remember, this is a design forum, so no code, get the Zagreus combat demo for a starter, though;-)
Alrighty, the first thing you will want to consider is moving the player. This is pretty simple on it's face. The Zagreus demo shows an easy way to do it: just have specific battle areas and search for an empty one. Of course, you will need to stick the enemy there, too. Now, also need some sort of routine so the combatants can harm one another. Check out Deadron's Event Loop for more info on this. You might also want to have different option availible by clicking on the enemy. See Clicked() in the reference(F1). After one party is defeated, you should probably check to see if it is the player and, if not, move him back to the map. Make sure to clear out the area either way so someone else can use it!

These are just starter tips. You may want to check out my DQO beta to see it in action (yes, I will update it someday:-).

-James