Fact: in this game, if you go to route 9, go to the haunted mansion area. Near the mansion will be a small house, go inside it. You will be able to get Giratina!
The only way I think is by finding a rockruff that already has it in the wild, since you can't find it as a hidden ability through breeding. But depending on which copy you're playing it might not be in the game yet (Dusk form)
I think the only way for now is to keep breeding until you get a HA on a scorbunny. Keep in mind that it's really rare to get a HA through breeding but the free ability charm from joining the game's group should help a bit
Join the game's roblox group then head to the house across from the pokecenter at silvent city. Talk to the man there and he'll give you a free eevee. I left the roblox group link in the description
I can't really say what the "best" is, but try Night Slash/Drain Punch/Swords Dance/Psyco Cut. Of course, it also depends on what kinda build you want for gallade