I got trouble with Primal dialga too, but seriously, like any pokemon game, It's all about the strategy.
Reviver Seeds are useful, because of Roar of Time, it would be helpful if you have 3 or more, I only had 2 and yet I still beat him, so basically, as long as you didn't faint in Roar of Time, you'll safe (Just make sure you have a LOT of oran berries.)
Actually, this is what I did. I throwed vile seed on Primal Dialga. Used Gravelerock, and when he comes near, use a sleep seed (optional) then use a move. Normal can inflict high-damage, but not so much. (Tackle is the only thing my partner used) anyways, use Water, Fire, Grass, whatever. Keep repeting it, Oh and make sure your partner won't use moves like growl and tail whip. Your Vile seed has taken care of your defensive needs, Also, eat oran berries if Dialga used Roar of Time, even when you think you don't need it.
Offence is the best defense, If you can give him a status, do it. (My ember inflicted burn on him.)
But, I'm still just a n00b, so.... I don't think I can be reliable at times like this, but it wouldn't hurt to try.
PS. I used time, it says in the manual, you can only get Vile Seed in time, so I don't know about Darkness.