Now that is a big question. If you have never built an enemy ai system before, this may turn out to be a bigger project than you are ready for. However, here is a very low level breakdown of the main ai mechanics of the basic beat'em up style enemies:
* Enemies try to get lined up with the player on the same y axis to attack the player
* Melee enemies try to get within x axis range while staying either above or below the player before matching the players y axis
* Single melee enemies will try to circle around behind the player to attack from the back
* Groups will try to surround the player
* Ranged enemies try to stay at a distance along the x axis but stay lined up with the players y so they have a direct shot
These 4 things are the major mechanics of the basic enemies in almost all beat'em up style games. While it doesn't include bosses or more difficult enemies, it is at least a start point. Of course, as I said, it is an extremely low level explanation and does not in any way describe how to pull it off in Construct. I want to stress again, that it would be a much easier task if you have experience with enemy ai systems. This shouldn't be the first type of ai you create.