I am hoping that someone will take this plugin request. I will give the dev all the credit for the plugin on my RP Server. Mobs spawn in places where you set them up, and they are easier than normal, like the zombies on MineZ. These mobs also have levels. You can config the rarity of the levels in the config. The higher the levels are, the stronger they are. (more health/protection,does more damage) Bosses: Bosses are rare, and really powerful. They have armor (can be configed) and a sword. If you kill a boss, it drops an item assigned to that kind of boss. Well, thats it.
Epicboss has stopped development. Hoping one of the contributors will get the torch passed into them.. But thehelper has been Mia for a couple of weeks now. Look up leveledmobs. It's almost exactly what you want. It really adds an rpg feel to a server. You can even set spawns for them to start and the levels start to raise as you go further. Check it out
Negative Batman. He abandoned it, via his facebook and him forums. But, lucky for us, one of his contributors has now working on a re-envision of it called MythicMobs. 3 cheers for Xikage!