Behold My True Form: Creating Evolving Boss Monsters for 5th Edition

You can also make a solo monster challenging by giving it multiple turns a round.

Say for example:

Monster
PC 1
PC 2
Monster
PC 3
PC 4
Monster
PC 5
PC 6

or

Monster
PC 1
PC 2
PC 3
Monster
PC 4
PC 5
PC 6

And use abilities that target multiple PCs at once.

I also made boss monsters/creatures that make sense for why they're able to do this.