Is the move earth power affective against Giratina in Pokemon diamond?

Answer 1

Normal effectiveness should apply if using Earth Power against Giratina in Diamond. If you need a super-effective move, use any strong Ghost, Ice, Dragon or Dark type moves. Giratina has a pretty high base stat for both Defense and Special Defense so whether you use a Physical or Special move shouldn't matter much unless you use some type of stat altering move to decrease one of it's stat bases, ie: Tail Whip and Screech will reduce your foe's Defense stat, Metal Sound reduces your foe's Special Defense stat. To this effect, Earth Power does have the added benefit of potentially reducing your foe's Special Defense stat while also causing damage.

Hope that helps!