Lol, I would own all of yal, I have all of the legendaries ;D
Most legendaries are banned from competing, thus rendering this a mute point. Also did you read this?
Its better to give Ratata Quick Attack (or any other move that makes you attack first), Endeavor and a Focus Sash. The Focus Sash will make it live with one HP (sometimes but it works enough to make this kind of thing a pain in the ass) then Endeavor will reduce the foe to one HP no matter how much HP, then you can use Quick Attack (or one of the others if you can get it) to win the fight. Ghosts live through this, but not even Legendarys are protected from it (Unless they use Protect :o).
Everything has a weakness and a pure Legendary team does not make you special, it just means you know how to use the strongest thing you see without any strategy or thought. The Focus Sash, Endeavor and priority moves can wipe out legendaries with no problems. Not to mention a well trained pseudo-legendary (Notable for their Legendary level stats, but are not banned from competing) is
ALWAYS better than Legendary pokemon beacuse you can breed the nature you want, perfect IVs and then choose the EVs you want to max out your team's power and type coverage.
So get a pokemon with endevour? Because my staravia has that.
No I wouldn't use it on staravia as it is more of a counter type of a move. Staravia is best for straight up attacking. Just try to get good type coverage (Try to limit the number of pokemon that Thunder is super effective against to one or two for example) or just pump your starter, or a pseudo-legendary full of EXP. At high levels pseudo-legendaries and the starters, even with ignoring IVs and EVs, can hold their own against the AI.
Here is a list of pokemon you can get that are ok or great to train (their evolutions have the real power).
Magikarp - (Water / Flying Evolution that is actually not that bad.)
Bronzor - (Look around Route 206, wayward cave has them.)
Happiny - (You get it from an egg, level up with an oval stone to evolve.)
Spiritomb - (Talk to people underground to unlock it, must be other players.)
Gible - (Look in the secret entry to wayward path, its under the cycling road.)
Riolu - (You get it in an egg from Iron Island.)
Feebas - (Good luck with this one only 4 per day. Max beauty to evolve it.)
Sneasel - (Late in the game found on Route 217.)
All but the last three can be found fairly early.
If you have access to people willing to trade anything, you could also go after:
Gengar
Alakazam
Golem
Machamp
Rhyperior - Trade a Rhydon that is holding a Protector.