Coming up with a balanced one-shot encounter for epic-level characters

[Teagana]

So I've been playing in a game since March, and the GM moved away during last summer, but he's coming back to visit soon, and our player-group is going to do a "one-shot week," where everyone we play with is going to run a one-shot in a different system. I'm doing 5e, because it's what I'm most familiar with, and I want to play with an epic legendary creature.

I thought about Tiamat or a Tarrasque against the 4 PCs at level 20(possibly + epic boons), but they know how to optimize and one of them thinks they can squish it at that point, using a half-dozen simulacrums with disintegrate...

So I'm rethinking it. I'm probably too new a GM for this, but I want to try, and it will probably go horribly wrong, but it's happening. Someone suggested I scale the levels down to 15, which is still incredibly powerful, and that apparently I can't trust the CR system, it's broken.

Anyone have suggestions for improved balancing math or player level/epic monster combos? I've forbidden wish, and I was looking at possibly using an ancient dragon, but I don't have any idea of the balancing math for it.

And the extent of my experience as a GM is a few tier 1 one-shots, and a campaign I started a few months ago that just hit level 4, it seems I'm always making encounters too easy for them, but I'm worried about over-correcting here.

[JNAProductions]

The CR system works fine. However, it assumes a 6-8 encounter adventuring day, which a one-shot boss fight is distinctly NOT. At that point, players can go ham and nova all they want, so...

If I'm allowed to toot my own horn, I think Argos (http://www.giantitp.com/forums/showthread.php?558845-Argos-The-Eternal-Frost-Two-Stage-Boss) might work well. Not sure exactly what level to have the players at, though.

[Gastronomie]

Do not trust CR math at high levels.

In my experience I've thrown a level 15 party an encounter with three times the adjusted XP of a deadly encounter and they survived just fine (a bit of a sweat). Here the monsters were approx. CR21, CR15 and CR 8, together with a bunch of CR2 mooks that appeared every round. But no one got unconscious, not even once.
These guys wipe the floor with Deadly encounters. Adult Dragons are like potato chips for them.

This will get worse at level 20. First trash the idea of solo because it will never work. Maybe throw in an Ancient Red Dragon on which a Death Knight is mounting, together with a bunch of summoned Devil or Demon minions, and finally we're talking.

And most importantly, try out testplaying it once. Two reasons. First is to determine the difficulty. Second is to understand how complex epic fights can become and sorta become used to it.

[Callak_Remier]

1) set the enviroment, inside a volcano, in the Abyss, Limbo. Someplace that sets its own rules.
2) Monster and Minions. Demogorgan and the 6 15th lvl spellcasters who summoned him. For example.
3) an Event that takes place mid combat.
Thanos Throwing a Moon at you kinda effect.

TPK or the Best 6 Hours encompassing a span of 30 seconds of in game time
You be the judge.

[Teagana]

The CR system works fine. However, it assumes a 6-8 encounter adventuring day, which a one-shot boss fight is distinctly NOT. At that point, players can go ham and nova all they want

That is a very good point, if they're bringing all their resources to bear, I'll take a look at that one.

[Teagana]

Do not trust CR math at high levels.

In my experience I've thrown a level 15 party an encounter with three times the adjusted XP of a deadly encounter and they survived just fine (a bit of a sweat). Here the monsters were approx. CR21, CR15 and CR 8, together with a bunch of CR2 mooks that appeared every round. But no one got unconscious, not even once.
These guys wipe the floor with Deadly encounters. Adult Dragons are like potato chips for them.

This will get worse at level 20. First trash the idea of solo because it will never work. Maybe throw in an Ancient Red Dragon on which a Death Knight is mounting, together with a bunch of summoned Devil or Demon minions, and finally we're talking.

And most importantly, try out testplaying it once. Two reasons. First is to determine the difficulty. Second is to understand how complex epic fights can become and sorta become used to it.

Thanks, I figured an adult dragon would be too easy, it'll have to be several enemies. How do you usually testplay encounters? With other players, or against yourself?

[Teagana]

1) set the enviroment, inside a volcano, in the Abyss, Limbo. Someplace that sets its own rules.
2) Monster and Minions. Demogorgan and the 6 15th lvl spellcasters who summoned him. For example.
3) an Event that takes place mid combat.
Thanos Throwing a Moon at you kinda effect.

TPK or the Best 6 Hours encompassing a span of 30 seconds of in game time
You be the judge.

Minions, minions may be necessary. I was just thinking it might be fun to have them start suddenly transported to the nine hells, or another plane. I like this idea. It also prevents the wizard from spending weeks and all his money making enough simulacra to double the party size. Or Limbo has weird gravity, doesn't it? Thanks, I like these.

[Laserlight]

The encounters my players have liked the most have had active terrain. It slides around (and you can hop a ride), it pulses with power, it's floating and you have to push it along or jump from piece to piece over a void, it has linked teleportals, open elevators and mine carts, that sort of thing.

[RSP]

To build off Laserlight’s terrain idea: at epic levels, straight up fights don’t work as well. Try to think of something outside the box, such as one one-shot I played in, the DM had us sealed in a room with the BBEG. The room had a turn (essentially lair action) one which a random effect would happen: walls would spring up dividing the party, or blocking character’s attacks; 9th level Dispel Magic (which really wasted some powerful spells and the turns spent casting them); shifting terrain; I think something else that I’m not recalling.

Basically, epic one-shots need something other than just a power villain, in my humble opinion.

Also, along the idea of play testing your battles, game plan your characters/players if possible. It’s a one-shot so there’s no reason not to prevent it from being too easy by tilting the table toward their weaknesses.

[Teagana]

To build off Laserlight’s terrain idea: at epic levels, straight up fights don’t work as well. Try to think of something outside the box, such as one one-shot I played in, the DM had us sealed in a room with the BBEG. The room had a turn (essentially lair action) one which a random effect would happen: walls would spring up dividing the party, or blocking character’s attacks; 9th level Dispel Magic (which really wasted some powerful spells and the turns spent casting them); shifting terrain; I think something else that I’m not recalling.

Basically, epic one-shots need something other than just a power villain, in my humble opinion.

Also, along the idea of play testing your battles, game plan your characters/players if possible. It’s a one-shot so there’s no reason not to prevent it from being too easy by tilting the table toward their weaknesses.

That's a very good point, I think so. Lair actions. I'll look into those. I've wanted to play with a wild magic zone for a while now, one of those might pop up, plus exploding lava pits, yes...