Um, that set is not very good at all. You don't want to have two moves that are the same type, especially if they are both physical. Draco meteor and dragon claw are okay together because Draco meteor is such a good attack, but I would recommend a completely different set that has done great for me. But it's for the origin form. It is:
252 HP 252 Atk 4 speed
Item: Griseous orb
Substitute
Shadow force
Dragon Claw
Earthquake
This set does really well for me. Giratina make really bulky subs, and has massive HP. Shadow force is pretty awesome on this set, as it does a large amount of damage to anything that doesn't resist it, and if blissey switches in for the immunity just use dragon claw after because it has no chance of breaking your sub on one hit. EQ gives you great coverage. Though it is kinda an awkward set, if it gets the chance to sub up It rips right through teams. Actually took out a whole team. And remember that if you use shadow force behind a sub, the opponent cannot touch your sub that turn. However, i cannot think of a good set for giratina altered besides a smogon set. Since it sounds like you want to use this as an attacker, i don't know. Giratina-A makes a better wall than an attacker, or as a calm mind sweeper.
252 HP 252 Atk 4 speed
Item: Griseous orb
Substitute
Shadow force
Dragon Claw
Earthquake
This set does really well for me. Giratina make really bulky subs, and has massive HP. Shadow force is pretty awesome on this set, as it does a large amount of damage to anything that doesn't resist it, and if blissey switches in for the immunity just use dragon claw after because it has no chance of breaking your sub on one hit. EQ gives you great coverage. Though it is kinda an awkward set, if it gets the chance to sub up It rips right through teams. Actually took out a whole team. And remember that if you use shadow force behind a sub, the opponent cannot touch your sub that turn. However, i cannot think of a good set for giratina altered besides a smogon set. Since it sounds like you want to use this as an attacker, i don't know. Giratina-A makes a better wall than an attacker, or as a calm mind sweeper.