You should divide your work. Beat'em Ups aren't simple are require a lot of specific and complex behaviors.
I am considering making a beat'em Up, so I already narrowed down some of the topics I'll have to work on. I'd probably do it on this order. Here's a list:
� Hero Movement
� Hero and enemy collision (when punching, kicking, check if attack hits)
� Hero and enemy health: drops when hit, plays death anim. when =0...
� Level: scene scrolling, enemy spawning events, screen locks until they are dead, etc...
� Common Enemy AI: When they move, attack (consider their at. range), drop their guard, retreat...
� Special Enemy AI: Bosses, sub bosses, etc. may have different speed, size, attacks...
� Combos and special attacks: grabbing and throwing, combo animations, etc...
� Any other additional feature: Specials, Animal Mounts, 2 player combos...
� Screens: Game over, Begin, Continue, Menus... The whole menu system.
It's a lot of features for you to work on. I'll be using this roadmap if I start to work on the Beat'em Up.