Yes there a few moves you can use: Haze after being used will turn off all stat effects meaning any increases and decreases will be removed and normal stats will be in effect, Psychic will occasionally lower the special stat after it inflicts damage.
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Calcium raises the Pokemon's Special Attack stat.
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!
You can find it on the second floor in the Veilstone Dept. Store. There is a woman in the middle who sells carbos She also sells Protein which raises the attack stat, iron which raises the defence stat, calcium which raises the special attack stat, zinc which raises the special defence stat , and HP up which raises the HP stat. Also there is a carbos in the gift shop place in the battle tower. I costs 1 battle point.