THIS!
Ready-to-Use Tactics for D&D 5E
themonstersknow.com
Your monsters are not stupid, they KNOW what they are doing. The above website/blog/book details tons of monsters and what their thoughts, ideas, plots, motivations are beyond the shit you read in the Monster Manual. Your monsters have survived this long to get to where they are not by being dumb buy by having survival and intellegence to know how to survive and flourish.
That site showed me what something so simple as a Fucking OOZES do in order to keep living day to day. We take that knowledge of human(oid) instinct for granted and say "Well it's just some orcs, they do +5 to hit and 1d8 damage with 15 AC." Fuck no they don't, they got where they are by being either bigger or smarter than their competition and have some base tactics besides "They swing their battleaxe to hit, oh they miss"
Giving your monsters battle tactics respective of their intelligence is a vastly overlooked trait.
I once had a pack of trained guard dogs flank the pc's and attack as a pack. You would have thought I made the goddamn dogs into Sun Tzu Clifford Edtion the way the PC's howled at my tactics. "Dogs wouldn't flank! They'd never do that! Why would they attack in front and back!‽?"
After combat I showed them a video of national geographic Hyenas in Africa and how they instinctively attack, feint, distract in a pack. Can't argue with Nature and training at bringing down foes bigger than themselves.
They still called Bullshit on me but everytime I threw in some canines they sure as hell took them out with a fireball first.