Some advice to how to get every legendary from every game.
Get your starter pokémon and breed as many as you can OR just breed one. Then look up other starters in the GTS from the same game or previous games. Then breed those two. If necessary, level and evolve them all. Then as soon as you have enough, deposit them in the GTS AND request more Ultra Beasts (particularly Celesteela because it’s the most requested one for some reason). Once you have the desired Ultra Beasts AND starter deposit them one by one and request the desired Legendary.
Legendaries that aren’t traded [yet]: Mew, Volcanion, Marshadow,
Legendaries that don’t appear on the GTS: Meloetta, Volcanion, Marshadow, Mew, Genesect, Shaymin, Deoxys
Legendaries that show up once in a while: Genesect, Meloetta
I hope this is helpful and please tell me your results! It works for me. And you’ll be lucky enough to get a SHINY too!