There's an Enemy Encounter script around on the internet for that (use Google). But let me tell you, it will take you a lot of time to get that script working properly. It took me a week of scripting and bug fixing to get groups of enemies to follow you when you engage one of them, etc. in an automatic system, and I only just finished it recently. X_x You'll also need a Pathfinding script, and have to edit the Enemy Encounter script to use the Pathfinding command for the enemy to return to its original location once the hero is far enough out of the view range of the enemy.