Units on this game are divided in 3 categories.

Attackers

Utility: Other Attackers get Attack set with Def set or crit damage set when they can use it (ex Lost vayne , zeldris , The one)

Utility units ALWAYS get HP set with Def set most of the times. These units are to exist support and NOT DEAL DAMAGE. If they have a barrier based on their attack it doesn't mean you need attack sets on them ( example Goddess Lizz, Green Merlin). Popular units in this category are : Goddess Lizz,

Green merlin, every gowther, red and blue king, Kyo ,Red zeldris (you can use hp crit damage set on him too) Green Drole

In the 3rd category we will put units that can get different gear and they are mostly pve units

For example you can use triple crit damage set on derieri when you want to deal insane damage on guild boss

Same goes to green king for gray demon.

If you have any questions about what category a unit belongs too just ask me.

P.S. I wanna continue this kind of posts to help players not get confused by others who give wrong advices.