Btw, this was first meant as tip to defeat Dialga, but can actually be used on all bosses =p

I've had a very easy time beating Dialga, I had Totodile as my starter and Treecko as my
partner.

Take some Stun Seeds.
In the beginning of the battle, line up with Dialga, then throw a Stun Seed at him, then
he won't be able to move until you attack him...
Then walk away as far away as you can, while still keeping a straight line with Dialga.
Then start using ranged attacks, like Hydro Pump or if you are Treecko, Bullet
Seed(<--which rules =P)
If he comes too close, just throw another Stun Seed and walk past him as far as you can
and just repeat...if you get hit by Roar of Time, just regenerate with A+B as he can't do
anything =D

Seriously, if you use Bullet Seed, you can defeat him in no time =p