Event I ENCOUNTERED THE STAR WARS BARBACLE TWICE BUT I COULDN'T CATCH IT EITHER TIME IM SO MAD

[Yoshiman5551]

Does anyone have advice or can just give me a 

[Auke1993]

Using Master Balls, if you have them, catches them instantly.
If you don't, I recommend taking some Pokémon with you that deal smaller amounts of damage. That way you can weaken it until it's at very low HP, after which you can catch it with Poké Balls or something like it.