and
Basically what you do is Berserker Blade 2 of their benched Pokemon for 40 damage with Hydreigon and with Darkrai/Cresselia LEGEND you use Moon's Invite to spread those damage counters on other Pokemon.
What else can make it better?
Captoats said:They both have massive retreat costs, so switching them out isn't really that easy. Also, even with DCE it takes a long time to build up Hydreigon. 60 on the active isn't that much and 40 on 2 bench isn't great. Sure, you have 140 to move around and (almost always) get a KO with, but it takes 2 turns to kill something that way unless you can KO basics on the bench, which is impractical to think about since it takes so long to power up Hydreigon in the first place. Their big hp's aren't really enough to make the idea worth while.