Honestly Impish is not a very good Nature for Lucario especially if you're play competitively and running a Special Set, as Impish increases Defense and decreases Special Attack.
Natures I'd recommend for Lucario:
Physical Set: Jolly (+Speed -Special Attack), Adamant (+Attack -Special Attack).
Special Set: Timid (+Speed -Attack), Modest (+Special Attack -Attack).
But since it's Shiny it up to you what you do with it, if you want my advice especially if you're playing competitively I'd try to get a Lucario with one of the Natures above.
Natures, EV's and IV's are much more important than a Pokémon being Shiny. As Shininess does nothing for a Pokémon other than change it's color.