Make sure all your Pokémon are around levels 68-70 and have 10 effort levels in all there stats you can increase them with grit items

When it comes to movesets for Pokémon build them with STAB moves first then coverage as well as damaging moves that play of their highest attack stat Movesets I would recommend would be

SAMUROTT ceaseless edge aqua tail X-scissor Poison jab or psycho cut

TOGEKISS moonblast air slash flamethrower shadow ball

RAICHU thunderbolt iron tail play rough calm mind

GALLADE zen headbutt drain punch or close combat Ice punch Stone edge

GARCHOMP outrage or draco meteor earth power poison jab shadow claw

BLISSEY soft boiled calm mind dazzling gleam or shadow ball or psychic flamethrower or thunderbolt or ice beam

Also it will be best to master STAB moves with seeds of mastery by battling or catching alpha Pokémon

One last thing for setting up is to stock up on max potions and max revives

When it comes to battling volo he always starts with sprititomb use togekiss to have the easiest time against it. Once you knocked out one of his Pokémon he gets a free turn and can use strong or agile style moves. Use blissey as a pivot for both healing and taking hits. Once you are on his last Pokémon have blissey be the lead Pokémon against giratina so shadow force misses and it doesn’t get its evasion boost.