Just fudge. I know it might seem dishonest, but I think I fudge at least a little bit in probably 80% of my encounters. The most important thing isn't accuracy from the DM's perspective, it's fun and accuracy from the players' perspective. Unless the players catch wind of the fudging, it literally only has upsides:
- You waste less prep time fine-tuning stat blocks and can spend more time on the interesting and material aspects of prep, like designing NPCs, dungeons, etc.
- It makes the combat encounters more interesting, because encounters that would be super one-sided in either direction are instead close and nail-biting
- It allows you to end a boring encounter quicker or prolong unexpectedly interesting encounters
- You can create cool moments where a monster fails an attack at a crucial moment or succeeds when the odds are stacked against them
...etc. I think well-executed fudging is a complete win-win situation, as long as your players don't find out.
Additionally I always setup encounters with a possible exploit/vulnerability that keenly observant players might notice which will significantly reduce the difficulty of the encounter. If they fail to figure it out, the encounter is far more challenging but still well within the player's capabilities.