I've done it where i have the tiles in my map decide if there's a random battle on the collision with the player.
that way all i have to do is have my players previous x and y saved temporarily, move him to his new x,y place enemies in there x and y placements then change the tiles animation by loading my battle map which is stored in the same binary format as any map in my game..
in my opinion easiest and least of a hassle cause everything can be done in one .ged and you only need to pass a couple variables, also the activation regions only cover wide spectrum areas to where as checking collision's with the player and tiles you can specify only certain tiles as enabling battles and others to not create battles i also only enable collisions on the specific tile i deal with collisions on so that way you pass minimal collision events
just my thoughts