Easy. Assuming your pokemon are around the same level as Giratina. Bring a move that can cause Sleep or Paralysis. If possible, bring a Pokemon with False Swipe, which is available at Veilstone Department Store as a TM. Bring Giratina's HP down with normal attacks, then, when its HP is low, use False Swipe. When it only has a few HP left, use the sleep or paralyze move, then use Dusk Balls (higher catch rate at night) or Ultra Balls. If the battle drags, try Timer Balls too.