hoot53

Best answer as chosen by Asker / Ratings
All you have to do is defeat it in battle. Your Pokemon should be level 53+ and have a type-advantage and Dialga will be toast.(Are you sure you don't want to catch it?)

stevew83

In-game you are much better off catching Dialga than merely defeating it. I caught mine using my Lv58 Empoleon and a Lv55 Noctowl w/hypnosis.I wore Dialga down with Empoleon (quite easy especially as Dialga has to rest the turn after it uses Roar of Time) and then put it to sleep with Noctowl to make it easier to catch. Try to save before the battle as if you accidently KO it without saving you will not be able to get another.

If you are trying to defeat it in a Wifi battle, it is weak to Fighting and Ground attacks so any strong attacks of these types from a Pokemon with decent stats should be able to take Dialga down. A helpful Toxic thrown in should speed things along nicely. Hope this helps :o)