D&D 5E Making Challenging Combats?

[LordEntrails]

So when in doubt, trade in big monsters for several monsters, or maybe have a single decent big monster as a center piece with a number of smaller monsters around.

Maybe, but this doesn't work with my group. They know to concentrate fire. Even if their is a boss, I always make sure they have at least one support that is almost as strong as them, if not two or three.

 

[MichaelSomething]

How you tried tried throwing a dozen Orges at your party?

 

[Retreater]

How you tried tried throwing a dozen Orges at your party?

I did a test fight on my own of (I believe) 8 hook horrors - which would be a similar challenge. A horde of large-sized creatures runs into the same issue: they are too big to effectively surround the group and get in their actions. Even if they split their movement by running in and attacking and then running back to give another enemy room to attack, they don't have the speed to pull back enough to allow multiple waves to do the same.
Creatures with flight could mitigate this, as would creatures with AoE attacks or decent ranged options.
However, I find it unusual to have difficulty challenging a 6th-level party (which hasn't been considered an especially high level in previous editions of the game). If these guys were 12th level, for instance, I'd not be as surprised.