I would prefer the answer to be for Black/White/Black 2/White 2,
But I don't mind if the answer is for another game i.e. soul silver/Heart gold
Thanks to whoever answers!
You can't in most cases without hacking.
You could get Palkia/ Dialga/ Giratina at level 1 via an Arceus event in HGSS, but otherwise there is no way to obtain any other level 1 legendary without hacking. The Arceus event has also long passed.