General

They're alright. Only plus about the shade is it's bind skill, Splitter, and no active buffs meaning no debuffs from bosses nor wasting time to buff up again.
When it comes to bossing, I would personally go with Dark they get almost everything Shades get but in an active form besides the binding and splitting skill. And you have Sacrifice + Gungnir which is basically an OP bossing skill. But this is coming from personal preference because I've gotten my Shade to 150, and it doesn't seem very appealing in terms of damage and boss. % boss is quite useless if you actual damage is low imo.

Basically what everyone said before me.

The splitter and death mark is really sweet, but spirit claw feels so slow compared to arrow blaster or frozen orb on my phantom.
I also hate mashing the button for the bomb punch, but that's gonna be changed in a future patch,

i find shade to be alright. maybe ts cause i think the hypers suck. damage +20%, boss att 20%, number of hits +1. they do have good bossing skills.just you need SI. decent SI is like having your hyper on all the time. and when you do have your hyper on (the +1 w.a speed and attack up) you become a beast. but idk, i just find bossing to be slow and alright personally, compared to my shadower.

@firster should add stepback and spirit ward. They are extremely useful for end game bosses, magnus/cra. And ofc the autorevive. I underestimated its link skill, it saved my butt a few number of time. I gotten some lucky proc, It proc 3 times in a row while I was stunned under cvellum's tail.

Probably in the top 5 in bossing

They're a pretty good class to boss with. With their cloning, bind and their Skills with Boss%.

That's all I got to say.

I think Shades are pretty good at bossing, because:
1. Spirit Trap
2. Soul Splitter
3. Death Mark
4. Spirit Claw hits so many times
5. Critical Insight, Spirit Bond 4

EDIT: Also what @RubberDonkey said:
6. Backstep
7. Spirit Ward
8. Revive skill (only 5 min cd)