I absolutely hated Primal Dialga the first time I fought him. Roar of Time always came at the most inconvenient moments and wiped my miserable corpse off his lawn. However, after three successful bouts with Primal Dialga, I can give you some tried-and-true paths to victory.
1. Use Seeds.
If you haven't figured it out for yourself by now, Orbs are useless in boss fights. However, Seeds are just as useful and I suggest you take a bunch of them. Sleep Seeds will buy you a few extra turns to heal yourself or your partner, or attack him while his guard's down. X-Eye Seeds will cause Dialga to miss all of his attacks 99% of the time and wander in random directions. However, this does not stop Roar of Time. Blast Seeds are great; eat them while right in front of Dialga and you will inflict a lot of damage, even moreso than some super effective attacks. Reviver Seeds are a given; bring around ten of these. The most useful seed, in my opinion, is the Stun Seed. Stun Seeds immobilize Dialga, who will remain that way until you attack him. This gives you time to heal, boost, get away, etc.
2. Hold Berries
Oran (and the rarer Sitrus) Berries are very useful in any fight, but a nifty feature that I think most people overlook is that, if you and your partner hold an Oran Berry, you/they will consume it automatically upon reaching critical health. Instead of taking 1-2 turns to heal, the first round is free.
3. Raise your IQ.
Raising your IQ with Gummis is super useful throughout the game. Certain traits, like Brick Tough, will make Dialga less challenging. Look here for a list of IQ bonuses and who gets them.
4. Be a cheap lil' sucker.
Use moves like Sand-Attack, Double Team, etc. to minimize Dialga's chance of hitting you. Use hax to your advantage and dodge like Spider-Man.
5. Stay determined!
You're bound to win eventually! Just keep practicing, and trying new strategies. Run out of items? Regroup at Treasure Town and stock up! Look for useful TMs like Brick Break and Earthquake to exploit Dialga's weaknesses. Don't give up.