Giratina can change into its origin forme by holding the griseous orb.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Griseous_Orb
As for the main question, I would recommend running a special attacking
Giratina because of its access to decent coverage and STAB such as:
- Earth Power
- Thunderbolt/Thunder
- Dragon Pulse
- Ancient Power
- Shadow Ball
- Energy Ball.
(Note: Giratina’s STAB also gets a further 20% boost in damage when holding the Griseous Orb.)
These are just some examples of Giratina’s special attacking movepool in platinum, you can check the rest from my source:
https://pokemondb.net/pokedex/giratina/moves/4
edit: Draco Meteor is also a move of note that Giratina has access to.
edit: Giratina’s signature move Shadow Force, while physical-attacking, is also also something to be noted so for this reason I wouldn’t recommend running a -Atk nature as even when uninvested, the damage can still be considerable.