In my case, I usually just stick to using Celebi, if I'm going to be using legendaries at all. It's good for setting up defensive/special defensive walls and--c'mon--it's cute as all hell.
I usually don't mind battling other players that use legendaries. Most of the time, a trainer that uses a six-legendary team is a 13 year old kid who hasn't even trained them properly, so they're normally easy to put down.
>>19279447However, I couldn't agree with you more. When I see that my opponent is using a Darkrai, I send out my fastest 'mons first and hope I can afflict it with some kind of status effect before it can get a dark void in. It usually busts through paralyses, though...