Largely I play a lot of monsters as semi intelligent, they make good decisions sometimes, but don't necessarily use their powers optimally in ways that only make sense in game mechanics, and they don't play with a perfect understanding of the player's power level. E.g. my Dragons won't focus it's a single person automatically because the Dragon doesn't have any way of knowing it can't just bat the player aside relatively effortlessly-- it'll thrash at multiple players to stomp them like bugs potentially multiple times, until it notices how resilient the players are, and since the battle is usually over in ~30 Seconds (4-6 rounds depending) it won't necessarily figure that out until its already sort of screwed or dead.
That said, I do have some monsters fight more like a pack of wolves, like Ghouls, where that 'feels' right or where the monster is more used to be potentially being an 'underdog' or can feel how powerful the players actually are-- like where they can tell they're outclassed enough to go all in on trying to take something else down or what have you-- conversely the dragon can tell its stronger than the players, just doesn't realize its not by enough not to underestimate them, because like, its a dragon and they tend to be arrogant and look down on adventurers.
