The weak VS the strong

[Jasdoif]

From the "simply fun" wizardry angle, a widened cloudkill cast by a level 20 wizard has the potential to leave a trail of corpses 40 feet wide and just over 2,000 feet long. A widened sunburst deals 6d6 to everyone in a 160-foot radius.

From the "simply fun" commoner angle, use extensive Aid Another for a Diplomacy check to convince a greater deity to move their godly realm to a spot where it covers the level-20s general area, then labor for a year and a day to apply a time trait where one year in the godly realm equates to one round in the rest of the Material Plane. With any luck the level-20s won't see any need to bother with the outside world, and in an hour and 15 minutes of their lives the commoners will outlive even a newborn elf with maximum lifespan under the accelerated time trait.