Easy!
For me it was so easy as beating a level 1 Wurmple with a level 100 Arceus, because my Dragonair and Torterra(both from Diamond) had own3d al his Pokemon(my Dragonair knowed Shock Wave and Thunder lol)
If you had a difficult battle you just have to build a better team and learn your Pokemon tricky moves like this:
Rhyperior-Shock Wave-for Water Types
-Flamethrower-for Grass and Steel types
Now this Rhyperior haven't one weakness at all!(well,only if the opponent pokemon uses Aqua Jet....)