Lat time I was DM (this past summer) I designed a two part "Boss Fight." The 6-7 PCs were eighth level.
It revolved around a Abyss vs Hell scenario. The PCs were trying to stop a hellish portal from infecting the land. (The entire area was sought after because of its ease of interdimensional travel - hence why demons and devils were fighting over the desert land. One side had a Lamia trying to open a portal to the Abyss, and the other a tiefling, trying to open a portal to Hell.
The Setup
The PCs had to make their way to a mine through a canyon or they could have entered through a secret cave in a dragon graveyard. They went the canyon route. After the canyon, they entered the mine. There, they encountered a boss.
The Good
The canyon fight went great! Some imps (used the Magmin stat block) as a distraction. A couple bearded devils on top of the canyon walls (forcing skill checks to get to them for some PCs, and allowing ranged attacks). And two Bone Devils, one blocking the entrance to the mine and the other flying from behind to surprise the PCs. The canyon. The enemies. The rear attack bone devil. The player feedback was solid.
The mine that housed the portal and tiefling had some good elements. It started with four separate trap encounters. They were solid, revolved around the theme of hell and a mine. And the PCs handled them well - as was expected. (They were already hurt from the canyon fight).
The Bad
There was another fight leading to the mine, a pack of hell hounds. It was supposed to be
in addition to the other encounters. They defeated the hell hounds, and two of the PCs had built an escape to rest with the lamia that I had not thought of. Therefore, they did that and then travelled to the mine. And despite the time function placed on the Hellish portal, it seemed inappropriate to punish clever play, especially clever play from five sessions ago.
The mine big bad boss fight had some problems as well. The tiefling was using the portal and the arcane structure to create a Nightmare. He had cut off the wings of the trapped Pegasus, and was in the process of the ritual. The PCs were supposed to encounter that and use resources to stop it. They didn't. I did not make it clear. They were on the hunt for the tiefling and ignored everything else - including one side room with a great trap!
In the final fight, the tiefling had a grip on a switch that would cave in the mine (There is a backstory to this; it is how his parents died.). This switch was like an old electricity switch, but it controlled gears that would collapse the mine. It was designed so the tiefling could appeal to the group and explain how the Lamia was actually more evil, and that this frontier land needed lawfulness. He also had an eighth level paladin,
one of the players, that was a paladin of Asmodeus. This was a secret. I mean the player let out hints, and it was such a short campaign, it never would have upset anyone, as it wasn't from left field. But one player caught on, and dimensioned door the paladin 500'
above the mine. They had an epic aerial battle.
The ranger/archer instantly destroyed the cave-in lever. He shot an arrow at it. I said there is a 10% chance it destroys it. He rolled, after hitting, a 98%. So the cave in was destroyed. They defeated the tiefling no problem. Like, they weren't even in danger.
(Now the aerial battle was incredible.)
So my faults were having a trap in a room that meant nothing, not having the Nightmare ritual have an effect (I wrote it, and the ritual did not finish until the next day after the PCs arrived - so I stuck to that), and doing the stupid cave-in lever.