Last night we ran a high level test, which ended up encompassing 2 battles over the course of about 4 hours. The PCs were a Lifer Cleric, a Monk, a Bard, a Paladin/Sorcerer and an Illusionist.
The (admittedly thin) backstory was horrible demons were attacking cities across the world and establishing magical energy pylons in order to eventually take down the Capital, etc... The PCs were Big Damn Heroes who, with the support of scrying and teleporting mages, could drop in on a group of demons and kill them before they established their pylon.
In the first battle (which did not include the illusionist as the player was not there yet) the opposition included 4 hezrou, a vrock, a barlgura and a Nalfeshnee. The paladin did huge amounts of damage every hit, using Shadowblade and Green Flame Blade along with his smite. The Life cleric ended up banishing all but the barlgura and the Nalfeshnee just a couple rounds in, so it turned into a pretty trivial fight. No one took any major damage.
Before they could do any resting, HQ called them to another battle (I wanted to see how much of their resources they had burned that first fight). This time there was an army of slaadi, which having killed the demons were trying to take the pylon as their own. There were 8 red, 2 gree, 1 gray and 1 death slaad. The illusionist was present for the this one.
The illusionist. Ugh. So, it appears that a high level illusionist can carry pretty much an infinite number of pre-cast illusions around in his pocket (or as jewelry in this case). Most high level illusions have unlimited durations. Illusionists have Maleable Illusion. So the illusionist casts the illusion, modifies it into a carryable object, carries it around until needed then modifies it back into an illusion of whatever. In addition to that, there is Illusory reality.
So the party drops in. The first round the illusionist creates an illusory adamantine shelter trapping the party and just a couple slaad, then makes it real. After a bit some slaad break through the walls of the thing and the illusionist casts mirage arcana to create a battlefield of difficult terrain and heavy fog except in an area directly around the players with a chasm separating them from the majority of the slaad force. I ruled that while the caster slaad could not see through the fog, as long as there were 3 living slaad within range of their telepathy they could triangulate area of effect spells into the PC safe zone. The illusionist player did not like this.
The monk showed promise but did not do much beyond punching and knocking slaad back or prone. The bard was moderately successful with vicious mockery and a few other spells. The paladin continued to do ridiculous damage, though the player was a little more conservative burning spell slots than the first fight. Eventually the illusionist had to drop the concealing fog (I ruled even the caster could not see through it because you cannot disbelieve mirage arcana) and the fight went a little faster once everyone could see everyone else. Eventually it was clear the slaad were doomed and the two that could plane shifted out. The party took almost no damage between easily made saves and counterspells and high ACs.
Overall, it was interesting but not especially fun. I think characters of that level would be more fun to DM for if you had gotten there with them, rather than made them as pre-gens. And I did not test any super high CR monsters because I wanted to know what a stock fight looked like. overall, it appears that while 5E is perfectly playable at that level, it isn't really the right fit for a convention game.
