Ok, so you switch in blissey, and now what? If it's the mixed version, darkrais sometimes carry sub so that they can just focus punch it, or it could have calm mind with life orb and actually 2ko blissey and ohko all the other randomly specific pokemon you mentioned. Gengar cannot even compare to what darkrai can do. As for deoxys-d....and what good use will a weak pokemon using extreme speed have? Superpower? That's a joke.
I'm just demonstrating how large Deoxy-D's movepool is. With Calm Mind and Bold Nature, it can really take some hits AND deal damage. Recover gives him his HP back while Shadow Ball, Grass Knot, or STAB Psychic can definitely kill some damage after 1 or two CM at base 70 SAtk.
"Gengar cannot even compare to what Darkrai can do." Isn't specific enough at all. Do what? 10% more accurate sleep on one Pokemon. If sleep clause isn't on the this thing is Uber no doubt. If there is sleep clause, then it sure is a fair game. Plus, after the sleep is taken, you send out a Heracross. What you going to do? Switch? Unless you carry Aerial Ace... And only you would be carrying that on a Darkrai... And even if you really do, Scizor can really wall Darkrai as well, same with Pinsir. Non-Mix Darkrai is even easier. Lucario shakes off its attacks like there's no tomorrow. Hidden Power Honchkrow with Insomnia can both take the attack as well as as kill it with HP Fighting, as long as it doesn't carry Ice Beam or Thunderbolt.
What you going to have? Dark Void/Dark Pulse/Sub/Focus Punch?
Ice Beam/Dark Void/Sub/Focus Punch?
Sub/Calm Mind/Dark Void/Dark Pulse?
There are ways to counter all of those sets.















