This is a little out of the flow, but I once caught Groudon with my level 15 Swablu.
Admittedly, Swabbz wasn't all that I used. But my numerous attempts at fighting Groudon either wiped me out, or wiped out Groudon, since two of my main party Pokemon were good against it. So I decided to put sneakiness into the procedure.
I sacrificed Mightyena and Swellow toward blinding Groudon. When they fainted, I threw out my Swablu. Groudon tried to KO it in one go, but since its accuracy was so lowered, it missed! I was like "OK SWABBZ YOU BETTER NOT MISS THIS" and commanded she use Sing.
She didn't miss.
Groudon fell asleep, and I quickly switched out Swabbz for Swampert. Swampert lowered Groudon's accuracy, and I threw an Ultra Ball. It worked.
Me: O_O
And this, dear fellows, is how you pwn a level 40 legendary with a level 15 Swablu.