I was scrolling through dice.camp and I saw a now-deleted post about a D&D 5e GM trying to figure out why their players were calling the last fight "too difficult". The confounding thing for the GM was the fact no one died. Nor had anyone even been knocked unconscious, though some got close. If anything it might have been a nail-biting brawl. But that isn't what players cared to bring up. Instead, the most memorable thing was the criticism about how it was run. In other words, the fight was boring. The players aren't having fun and they want to move on to the next thing. But combat is standing in their way. That is what I heard from that post.
I don't believe in a fight that is too difficult. Some of the best combats I have run and played in were tuned to be extremely hard if we just go by numbers. But what those encounters did was serve the narrative that was being weaved. They instilled fear into the travelers of the dark forest, made heroes feel powerful by taking on hoards, and set tension as the players fought atop a tower in a lightning storm. If we just focus on challenge ratings, character abilities, and damage per round, the only thing made is an awfully pretty math equation. Making problematic combats that, when run, will be a difficult slog and means boring experiences for players.
Now for the big question, how do correct this? Well first;
Avoid Hit Point slogs at all costs. Especially if the creature is difficult to hit in the first place. We want things moving quickly and if it doesn't feel like they are making progress things become repetitive and boring. Monsters should be easy to hit and take a lot of hits, or the reverse.
Make fights dynamic. Environments should have features our players AND our monsters can interact with to change up the fight. We want to avoid any creature, player or otherwise, doing the same thing turn after turn. Even if it is just having to move or turn attention to a new target.
Keep combat 4 rounds or under. Unless it is a huge set-piece fight that you've been building up to, things start to get stale after 4 turns. This ties into keeping things dynamic, as most times both sides of the engagement start to run out of new things to do after 4 turns.
Vary types of combat. Combats are supposed to serve the narrative. But there is such a thing as nailing a point too far down. Your horror campaigns need upbeat fights to break up the mental strain of the world. Your heroes need to be brought down a peg from the heights they've climbed. The challenged adventurers need to be able to feel like their skills can flourish sometimes. Let them.
Let our players be awesome. A little more of an advanced tip, but tee up the players for great moments. Let enemies group up for explosive attacks or the heavy hitters go toe to toe with the big fighters. It lets our players feel validated and it is an easy way to change up the combat if the enemies suddenly "learn" to avoid our player's strengths.
There are so many resources to help you build amazing monsters and layered environments to fight in. Most much more useful than mine. But if we just keep to these basic tenants our games will elevate tremendously