There is 1 easy way to 1 shot him with only 1 pokemon ! Make sure to have 1 gengar lvl 70-80 with speed stat gained from ability and ev trained ! Gengars speed stat is greater than giratinas speed stat at least on paper ! That should make you move first ! Then teach Gengar destiny bond ( a move that upon killing your pokemon takes down the opponents pokemon aswell ) ! Use destiny bond and after that Giratina should 1 shot your Gengar but destiny bond will kill Giratina aswell ! Keep in mind that you need to have at least 1 more poke in your team so you dont black out !
There's different ways to defeat Giratina at distortion world:
- Train a Skuntank to lvl 70 and learn toxic and dark moves gain type.
- Use a Chansey or a Blissey and use toxic+protect and SoftBoild and gain time.
- Use any Pokemon that is not weak against Dark / Ghost and use sucker punch.
- Ask for a Sinnoh pokémon to lent and use it against it.(Staraptor is a great option because almost everyone have one).