Just idle thoughts. I want to follow S'mon's advice about having my epic players interacting and depending on very weak NPCs, and I thought of a method that might work. What about a monster that has scaling AC and BAB based on his opponent at the time?
Say this monster has the special ability "Is not automatically hit on a natural 20" and then AC equal to: 10 base + 5 natural + 2x Attacker's Attack Bonus (or BAB, whichever is higher (incase of powerattack))
Look at how that works out:
Attack Bonus of 0 vs AC 15, hits on 16-20
Attack Bonus of 1 vs AC 17, hits on 17-20
Attack Bonus of 2 vs AC 19, hits on 18-20
Attack Bonus of 3 vs AC 21, hits on 19-20
Attack Bonus of 4 vs AC 23, hits only on nat 20
Attack Bonus of 4 or more... never hits.
Now then, give it magical immunity, and other things to resist high-level folks, and you have something that seems custom designed to harm the big guns. The gods themselves can't harm this thing, and if you likewise make it so that its attack bonus is equal to 2x(Opponent's AC - 10) and bypasses all DR... you have something that can kill the gods themselves, and yet is vulnerable to a single level 1 warrior.
Now, obviously, if this critter has a huge amount of hit points, the level 1 warrior is -still- going to have some difficulty killing the nasty moster. But that's where the epic guys come in.... guarding and helping the little dude survive, making sure that the little guy doesn't get too big, but making sure he isn't too small either. An interesting challenge, no? I guess there'd have to be some reason they can't just get an army of peasants as well...
What do you think?