Fun, unique, and/or challenging combat encounters

My philosophy is simple. Every enemy team should have one or both of the following:

• An overwhelming, but predictable attack. This is something with nearly a 100% chance of harming the targets and that deals serious harm, but that can be anticipated at least 1 round in advance. This gives players the ability and responsibility to take cover, apply buffs/nerfs, or prevent the attack. For example:
  • A dragon can use its breath weapon an unlimited number of times, but its mouth begins fuming 1 round before it exhales
  • A giant has between +10 and +17 to hit, deals 30 or so damage per hit, and sends its target flying, but must choose a target and lift its weapon 1 round before it attacks
  • A griffon swoops down to grapple a creature with its talons one round, dealing a little damage, then Dashes up to drop the creature from 80 feet in the air, dealing 8d6 (28) damage and knocking then prone
  • A sniper has the power to aim an attack as an action. As long as his concentration is not broken and he does not lose sight of his target, he fires the attack at start of his next turn, automatically scoring critical hit. That's 2d10+15 (26) for a heavy crossbow with Sharpshooter
• A robust, but imperfect defense. This is something that makes conventional attacks useless unless a certain condition is met. This gives players the ability and responsibility to puzzle through the enemy's defenses and defeat it in just a few hard-earned attacks. For example:
  • A dragon has a feature called "Invulnerable Hide" that raises its AC to 30 and gives it evasion, as well as a feature called "Soft Underbelly" that let's you ignore the evasion and treat the AC as 10 when its belly is exposed (e.g. when it is prone or flying)
  • A giant is 50 feet tall with a feature called "Elevated Weak Point" that lets it ignore effects that don't reach at least 25 feet up unless it is prone, as well as a feature called "Swat Missiles" that lets it treat its hands as 3/4 cover against ranged and area effects unless it is incapacitated or restrained.
  • A griffon has a feature called "Flyby" which means it doesnt provoke opportunity attacks while flying, and a reaction called "Barrel Roll" that boosts its AC against ranged attacks. Readied actions to grapple it when it's near the ground and effects to knock it out of the air are your friends!
  • A sniper is 600 goddamned feet away, but helpless once you close in.

This way, every combat is a puzzle, and not just a slog through hundreds of hit points. The combat challenges the players to understand new problems and come up with creative solutions with whatever features they happen to have.

When designing a combat encounter, try to consider the terrain on which the combat will take place, as well as the current weather; both factors can add to the fun and uniqueness of an event. For example:

1. On top of a tower, during a windstorm.

2. On a speljammer, during a hailstorm.

3. Underground, during an earthquake.

4. On a hilltop, during a mudslide.

5. On a watermill, during a downpour.

6. On a moving chariot, during a sandstorm.

I've done a gauntlet-type for a session, with some sort of time limit.

The party found the necromancer (BBEG at the time) doing a ritual, surrounded by several followers. My intention was to not let the party fight the necromancer directly, since it was way over their level at the time being, so I sent the followers to distract them while he continued the ritual. They had to stop it in time or something bad would happen.

The followers were weak af, and that was intended. They quickly dispatched them, but as soon as the all died, the necromancer told the party "thank you, I needed stronger minions anyway" and brought them all back to life as zombies/ghouls.

The collective "Fuck" was *chef's kiss*. When they finished all the minions, some RP happened between the necromancer and some other BBEG, which caused the ritual to fail and summon a Flesh Golem.

The golem by itself was meh at best, but since they had spent some spell slots already because of the previous fights and were a bit battered up, it ended being a really interesting fight.

When the session ends, I like to ask the players if they enjoyed it and they absolutely loved the gauntlet fight.

I'm relatively new at DMing, but I'm a sucker for nonlinear fights. I love to test the players with though but fair fights, that aren't just "enter the room, kill the boss".