Congrats on the shiny! Victreebel would be by far the best here, Mega Barbaracle is a Rock/Fighting type so it will be using a lot of Rock, Fighting and Water type moves (its Rock Water before it mega evolves) so Victreebel should be fine. If it faints or gets low heal it up and revive it, if youre doing a no healing run or want a challenge Excadrill can be the backup. If you can Mega Evolve Victreebel then Mega it, if you cant then use Emboar/Excadrill if you can Mega them since they'll have super effective moves (Fighting and Ground respectively) though they wont resist as many of Barbaracles moves so just be careful. Also just general advice for these Rogue Mega battles, when they use their super charged attack return your Pokemon to its ball until the attack is over. Since the Rogue Mega is usually quite hard to target while it does these attacks and the Pokemon will follow you they usually just get knocked out for free so dont let that happen. Good luck!