Game Difficulty Problems... need suggestions.

My adventure/rpg map i'm making will be 1 to 4 players, but I'm kind of limited on idea's on how to adjust the difficulty.

So far, I plan to have all heroes share experience and balance abilities with this in mind. This way, if one player kills x amount of mobs to attain level 10, then two players will both be level 5 after x amount of kills. To do this, I'm going to scale the abilities so that every level of a spell adds the same amount of power, IE, level 6 will be twice as powerful as 3, level 10 twice as powerful as its level 5 counterpart.

I'm sure some of you know Enfo's Hero siege do this and i'm sure other maps, but what other difficulty adjustment systems are there?

Edit: Also, for my map, just adding more creeps doesn't sound good, keep that in mind.

Make the Creeps harder to kill? (More HP,DMG,Exp etc.)

hp handycap. this methos has been used since the original cube d days.