DMing 4ED I would ignore the DMG challenge approach. It doesn't make sense a small town having lv 10 guards just because the players are lv 10.
But if you're married to the idea that guards are only good for beating up peasants and a PC of 10th level is TEH AWESOME!!!!!1!! then obviously you're not going to have your 10th level party fighting guards.
But there's a long way to go from 10th level to 30th.
Well it again depends on how you view level 10. Since a level is abstract in one person's campaign a level 10 is the equivalent in terms of the sense of power in that world to a level 1 in another campaign.The idea of a town where the guards can provide a credible challenge to a 10th level party is pretty frightening to me. It would be like one of those kung fu movies where it turns out the fish seller can break pillars with his bare hands.
Exactly.Guards should be at least as powerful as the opposition they're supposed to guard against on a day-to-day basis; otherwise the opposition, rather than the guards, would control the town.
..."Shrödinger's stats"...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.