Yeah, I'm not big on fudging rolls, but that's one thing I will do. In my last campaign, I had statted up the first real villain for my players to fight, and they knocked him out in one punch. I would have made him one level higher, but then his own attacks would have been strong enough to one-shot some of the players. Level 1 woes.
Yeah, I learned that too. I had come up with a villain later on who had a very defense/counterattack focused stationary fighting style combined with sundering armor, and I thought I could make him a big threat, but then he ended up completely flopping because there just wasn't support for building that style and making it strong. Now I'm playing looser, and stealing lair actions from D&D (minus the lair part most of the time) to make my loner villains work.
There is a reason why the D&D 5e creatures have their HP written in dice values (4d6+10).
It allows for variation within the stat block. But it also gives a maximum and a minimum HP they can have.
Most of the time you use the average. But if the game is too slow, you can lower it to the minimum HP. And if they are steamrolling an encounter, you can just increase the HP to the maximum.