So right now I'm reading through the Sea Reavers adventure from today's Dungeon article. Not to give away any spoilers, but there are some high level (15th) human enemies. How do you justify random guards, assassins, etc. or whatnot being above, say 6th or maybe 8th level? (In 4E terms; adjust the level/power based on your system of choice.)
Generally speaking, I don't. I despise that crap and always have. If you don't want high-level PCs being able to wipe the floor with the city guard, then don't use a game system where the PCs' combat power increases exponentially with level. (Or else ratchet down the PCs' advancement rate, or cap their advancement at a point you're comfortable with.)
In my campaigns, random guards are mooks. Previous to 4E, they were in the level 1-3 range. Now that we have the minion rules, they can be up to level 8-9 minions (but that's for very tough, disciplined soldiers; remember that legion devils start at 6, and any fighting force tougher and more disciplined than legion devils has to be pretty bad-ass).
To me, this is vital for maintaining immersion. Players have to have a sense of how powerful their PCs are in the grand scheme of things. If you're a 15th-level PC, you should be able to say confidently that you can carve your way through a bunch of guardsmen without breaking a sweat. The guardsmen should not suddenly shoot up ten levels because some paragon-tier PCs wandered through town. I would mercilessly mock any DM who pulled that kind of baloney.
Now, that doesn't mean PCs will never face high-level NPC foes. But when they run up against such enemies, there should be a definite sense that they are facing people who are really special - like the PCs themselves. These folks are not random guards or wandering thugs. They are important characters with reputations, maybe even legends.
I remember when I was playing the first expansion pack to NWN2, I got into a bar fight. My party consisted of four or five 19th-level characters (and remember, this was under 3.5 rules). Properly speaking, a bar fight at that level in 3.X ought to go something like this:
"The fighter sends the thug and fifteen of his buddies sailing across the room with one swing of his barstool, looking rather like Sauron in the movie version of
Fellowship of the Ring. The cleric utters a brief prayer and everyone in the bar is dazzled by a flare of holy light, leaving them stunned. Then the wizard wiggles a finger and the building explodes. Meanwhile, the rogue has picked the pockets of everyone in the room, emptied the till, and found the tavern owner's secret stash under the floorboards."
Instead I found my party of supreme bad-ass heroes having a rough time with a bunch of lowlifes in a bar. It totally yanked me out of the game world and killed my interest in the story (not permanently, but enough that I quit out of the game and didn't get around to starting up again for a while).
