D&D is not a videogame
Monsters don't exist only to die, monsters aren't punching bags, monsters aren't a handful of lines of AI. You, the DM, are responsible for making these monsters behave in the way you see fit. Strategy is only meaningful when there are multiple valid choices.
A monster at low hp is less effective than one at full hp.
To a monster hp is more than just a number. It is their morale, their courage, their lifeblood. If you take away half of a monster's hp, you have shaken their resolve and scarred their body. The way they act, the way the DM controls them, should fundamentally change.
I'm sure you've seen in movies a scene where a tough guy gets wounded, and he gets angry. You've also seen someone wounded and even a drop of blood turns them into a blubbering mess. How will your monsters react to hp loss? Does it drive them to anger, do they flee after 1 damage? How do their friends, family, comrades, clan take it? What stories do they tell at the monster tavern after the fight?
What do your monsters feel when attacked? What do they want? What don't they want?
Let's slow down and think about an example for a minute. Your party of 4, Wizard, Rogue, Fighter, Druid, happen upon a band of goblins. Immediately the party sets to work all attacking 1 particular goblin.
- What do the monsters feel? Fear, anger
- What do they want? To survive
- What don't they want? For themselves or their comrades to die
So how should the goblins react? Firstly, the one being targeted will probably flee. They will try and get out of line of sight, act defensively, stop the pain and prevent imminent death.
What will the others do? Act to defend their goblin friend? Block line of sight, cast defensive magic, distract the party, physically prevent the adventurers from pursing. Will they go on the offense? Will they target the weak wizard, will they flee or regroup?
This fight may play out as follows:
- The two forces meet, and the party immediately focus fire 1 goblin.
- That goblin flees, hides behind a box
- The rest of the goblins jump in front of them, some of the goblin rogues flank the wizard, the ranged goblins target the rogue. One goblin begins to flee to alert others.
- The party is now forced to react piecemeal. They can't communicate smoothly in combat. The wizard wants to defend themselves, the rogue disengages top avoid damage. Will the fighter pursue the fleeing goblin? Who will the Druid support?
The party now has options. Their enemy has reacted and taken away easy choices. Force them to act fast, always change up their priorities and thwart their plans.
Whatever the party chooses, the goblins won't make it easy for them. Once they have taken some wounds, the goblins retreat, they won't risk their lives for nothing. However, they will hold a grudge for free. In the future these goblins could harass the party at night, or from afar, preventing them from resting and healing.