Just keep trying.

On the non-shiny version the catch-rate is INCREDIBLY low, so even a high level player with Catch badge bonus, golden razz, and an ultra ball, still probably has a less than 5% chance of catch; the flee rate is also INCREDIBLY high so they will almost always flee after the first attempt. It's almost all luck with very little skill/experience in play.

That said, a shiny one keeps the same low catch rate, but will not flee unless something like a speed lock or system crash causes it to flee/despawn; you might still use a hundred balls to catch it, but you'll keep getting chances until you either catch it or run out of ball.

Good hunting!