And my point was that there are Pokemon that can be adapted to suit whatever the situation is. Likewise, depending on the situation for the attacker, certain Pokemon won't work. Anyways, if Darkrai is being forced to switch, then you can pick at with through residual damage, and if Darkrai is fleeing, it's not attacking and thus not accomplishing anything.
And I never said Scarf Gengar was a Starmie counter. In fact, I never mentioned the Choice Scarf at all.
Again, I specifically stated that Heracross could be useful for Darkrai that don't carry psychic. If Darkrai is carrying psychic, then it's not carrying a different move, and thus it can be handled in a different way.