So a few weeks ago I tried my hand at being a DM for a level 20 session. It did not go as planned. But my friend Jay [who was one of our 2 DMs at college] sent me a PM with some tips that I'll share here.

Few words of advice for one shots and general advice

1. Don't have anyone in the party that's destined to kill one another. It kind of just complicates things and if you don't have a very complex and built up story, it's iffy. Fun, but iffy.
2. If you know it's a bunch of high level PC's versus a spellcaster, let said spellcaster open up with something HARD. Example: Vox Machina being all level 15-16 (7 PC's, 2 NPC's, 1 guest) but the opponent being an Ancient Green Dragon that was also a 9th level spellcaster that immediately opened up with Meteor Swarm
3. If you know the session's going on late and/or might run late, only buff enemy damage
4. Instead of trying to calm everyone down forcefully, just wait calmly and use it to your advantage to think of your plan to attack. Or add temp HP. I'll openly admit when somebody says and or does something completely out of character/only escalates a situation to where force and/or violence seems to be the answer, I'll usually stack on some. For Maeve's case she only had 30 temp because she already had resistances to most things.
5. Make it a point that everyone goes ONE at a time. Small talk/whispers for strategizing are okay but if there's a full on conversation where people are begging for things to happen, it's okay to let SOME things slide, but don't let everything slide. For example, I wouldn't have given Chandler advantage for being next to Ben because he already had a good +13-15 to hit anyway so he can roll low and still have a high chance to hit. Plus, Vow of Enmity was a thing but he forgot.
6. Lair actions are always good.
7. Always prepare for the worst/best of the party and have something that's easily interchangeable in case something goes off the rails
8. Never make things set in stone. Make things seem more like guidelines/branching paths so if the party does something you know what to do after the fact.