20th level villains have tiers of henchmen. It's like a pyramid scheme, so that they might have thousands of 1st and 2nd level mooks around the country. Occasionally, those mooks fail to kill off the 'heroes' who show up in the same area, and thus the heroes draw the ire of the next tier of the pyramid. Most villains assume that their henchmen will take care of the heroes because there are lots of potential heroes who do get killed before they become a threat. PCs are just wild cards that mess up their calculations, and who are able to climb the pyramid to the leader.
Monsters, though? I don't use that many monsters. Most civilized lands don't have monsters, so you only fight them when you go to exotic locales like other dimensions, or the underdark, or the main villain's fortress.
In my last campaign, the plot went something like this:
Start at 5th level. Begin by fleeing the country because you're being hunted by the establishment, and they're much more dangerous than you.
Get to 6th level, join forces with a rebellion. Fight the scouts the evil empire sends to check on your rebellion.
Get to 7th level, head out to find friends for the rebellion, but along the way run into the evil empire forces that had been sent to destroy those potential friends. Fight them.
Get to 8th level, and start snooping on the secrets of the evil empire. The empire realizes you're a threat, but you're nowhere near as dangerous as some of their other enemies, so they send a group of 12th level folks your way to take you out. Through the aid of unexpected allies, and the amazing good fortune of convincing one of the badguys to joing your side, you're able to escape.
Get to 9th level, and help defend the rebellion's base from a major attack. Using your knowledge of the secrets of the evil empire, set out to undermine their power base. Get captured, and count your blessings that the evil empire's warlord doesn't kill you outright. Never suspect that you're actually pawns in her game.
Get to 10th level, and join forces with a bad guy who is the lesser of two evils. With a bit of aid from him, undertake a daring journey to destroy the sources of the evil empress's power. Realize you've walked into a trap, but again thanks to remarkable luck and a bit of divine intervention, take her down, trapping her forever in a giant cave-in. Through cunning political positioning, convince the dissidents in the empire's own army to join your side, and turn your efforts upon toppling the last remaining remnants of the empire.
Get to 11th level. Realize that your 'ally' bad guy who used to be the lesser of two evils is now the greater, and try to take him out by allying with yet another bad guy who is the new lesser of two evils. But this new evil ally betrays you, and the major bad guy tries to destroy the rebellion. Take your army and kick his ass.
Get to 12th level. Having lost most of your army kicking the ass of the previous major villain, the third-string villain who was biding his time all this time becomes your major foe. With few allies to rely on, you chase after him, falling prey to traps and ambushes occasionally, forces he has sent your way to slow you down as he tries to tap the same source of power that you never quite managed to destroy way back when you killed off the evil empress. Realize that the villain has a lot more resources than you, and that even if you spend time preparing, he'll spend the same time preparing and will get more done, so your best bet is to attack him now. Attack him and realize that he was a lot closer to his goal than you thought, so it was a good thing you attacked him now, but thankfully you do catch him off guard, and in a metaphorically appropriate way, defeat him. Save the day.
Get to 13th level. Campaign ends.