Need help with a 20th lv oneshot I'm going to run.

[Sir cryosin]

So my DM might not be able to come to are next game. I said I can run a one shot if they want. I dm for a different group. They said yeah what lv? So they asked can we play 20th lv and I said sure. So I have no experience with this high of a lv play. I'M excited to run it because I get to use the cool monsters and see what the party comes up with but. I don't know how to make and encounter for this. The party it's talking about a ranger rogue that wants to use volly so lost of minions. And the other a bladesinger then a kitsune somekind of minion summor control that what she likes to play. Then there are 3 more people that I don't know what they are going to play.

[MrFahrenheit]

So is this gonna be a true "session," or just one combat?

If the former, give the party one short and one long rest, but he sure the latter takes place early on, to make a battle vs. solo boss more epic. If the latter, you need three kinds of enemies:
1. BBEG. A lich, ancient dragon, etc.
2. What I like to call the "flanker." You may even want two of these, depending on party composition and/or size - mid/high CR, though definitely a few steps lower than the BBEG. What they do is attempt to get around the party tank(s) and get at the casters.
3. Henchmen. These don't have to be CR 1s. At this level, a handful of stone giants are an easy choice. They serve a two-fold purpose: first, they protect the BBEG. Second, they tie down the party tanks from protecting against the flankers.

[Ninja_Prawn]

How many encounters do you expect to have time for? It would be a good idea to build a variety of challenges, as that should let different players come to the fore. With a large group of 20th-level characters, it's fair to assume they can handle pretty much anything you want to throw at them, so mix it up as much as you can. Hordes, fliers, spellcasters, lair actions, interactive environments... whatever you can think of.

[Sir cryosin]

I was thinking they fight there way throw a city. Mowing down minions. Trying to find the boss in between there will be couple of skill challenges and traps finishing up with the big boss fight.