Unfortunately, I think Joker's the best, with Tank Mega Diancie and Celebi as the spirit team. I think Mega D has the highest defense of any 3 slot spirit, which reduces your damage and increases the effectiveness of Celebi's AutoHeal. Rebel's Guard is great to armor through many boss's attacks.
Not dying is a pretty big priority in these boss battles, so a heavyweight will be more effective than a featherweight. Bowser or King K. Rool would be great for that. Bowser's nair has a sweetspot that can do a ton of damage.
The best form of survivability would be some for of healing, but I don't think Dharkon uses anything classified as an energy projectile, so the absorbers (Ness, Lucas, and Mii Gunner) aren't as useful there.
