How do I defeat diagla

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?)

Defeat Dialga? Why on Earth would you want to defeat him, most people only want to capture Dialga, not defeat him. But hey, it is your choice. Dialga is only on level 47 so by that time he should not be that hard. Your Pokemon should be above level 50 or so, so he should not give you much of a challenge. Keep attacking hi, and heal when you need to, if you even need to. Legendary Pokemon are good, but when fighting them for the first time, they suck. If you do feel like catching the Legendary Pokemon Dialga then weaken him till it's health bar is red and keep throwing ultra balls at it. Use a move like Thunder Wave, or Hypnosis. That way, you can either paralyze him or put him to sleep. Hope this helps.

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)