D&D 5E DM Help: Tips and Tricks for Monsters In Combat

[Buttmuncher Bob]

When it comes to flying characters with missile fire that refuse to come within range of a land bound monster, use basic tactics of enticement and deception. Take for example, a dire troll hunted by an assassin making ranged attacks from a broom of flying. Troll takes a few hits and then falls to the ground gripping an arrow/quarrel in it's chest, feigning unconsciousness until either it's attacker is satisfied and leaves or, the assassin comes to take a part of it as proof of the kill or to simply collect unbroken ammunition at which point the dire troll uses half it's movement to stand then proceed to make it's five attacks against it's would be killer.

 

[Dan Chernozub]

I think I’ve seen such thread before

Low level NPC casters. No need to fireball the PCs, players might not like it. Don’t go flashy, go nasty.
Use debuffs. Use obscurement. Use heal on the fallen (as a DM you get to decide if monsters die at 0 hp - I make it clear to PCs that in my games they can never be sure if it is really dead unless I explicitly tell them so).
Buffs are rarely worth it due to the nature of monsters being more tanky than dps-y.
Spend time to choose what is the bad little guy going to concentrate on.
Choose something doesn’t give a safe every turn and synergizes with the rest of his group.

Example: add a WarPriest to you Hobgoblin patrols.
Choose Bane to concentrate on and complement the high AC of your frontline.
Give him Healing Word and Aid to bring them back if the enemies manage to bring/blast them down.
Add Command, and time the attacks so the rest of his buddies attack the BSF who just dropped prone because Wisdom is viable dump stat.
Toss in a Spiritual Weapon to optimize the resources (Bonus Action, no concentration).

Also a good way to show new PCs some tricks.