As a GM I’d like to feel like I fully understand how to utilize my monsters tactically, but over the past couple months as I’ve been running the beginner box and now leading into Abomination Vaults, I feel as though I’m very often just making three strikes with an enemy or moving and striking twice unless the enemy has a very specific action in its statblock which is obviously better. Like a dragon is obviously supposed to use its breath weapon, but then I ran goblin warriors last night and I couldn’t even figure out how to use their scurry reaction beneficially.
Monsters can use skills too, so look at what skills they have and what basic/trained actions they have access to. Disarm is a joke until an enemy disarma the fighter and then attacks their sword, destroying it (since it’s an unattended object). Also, your players might hate you if you do that.
Move around, if you are going to make a 3rd strike it’s probably better to move away unless the player has an Attack of Opportunity or similar, and even then you can just step away.
Use terrain more. The mosters are usually on the defensive in my games, so they might have traps, advantageous terrain, etc. If the enemies are shooting from a balcony then they are not in melee range, and probably can Take Cover each round. If the usually melee PCs made the mistake of not carrying a ranged weapon they’ll need to either find the stair up or climb which is not ideal.
Use (magical/alchemical) items! There’s quite a long list of them and I feel they are underused in the game, my players at least have this “I might need it later” mentality, specially (but not only!) towards consumables. I personally blame Final Fantasy for that, oh well.
Basically any advice that works for the PCs is going to work for the enemies tactics-wise.