Don't worry Dialga is easy, you need to be at least: Lv.62
(I used golden seeds that raises you up 5 Levels at a time!!!!)
All you need is one of your starting pokemon to know Dig, and one that knows Detect (Not really needed but,It will protect your partner).
Keep using dig and Detect to Defeat Dialga (If you need the wondermail codes for Golden seeds,I will post them soon!!)
[Edited by Wigglituff, 10/29/2008 1:04:02 AM]
I was only a level 40 when I recruited Dialga...