Maybe it was just me but I didn't find this fight nearly as hard as many claim it to be. Meh I don't know why. Anyway Gloomtail is much like Hooktail just a far stronger version. Anyway the usual attacks such as Power Bounce Power Smash and Ultra Smash will work very well in this fight while Vivian or Bobbery are very much recommended since they'll rake up the damage quickly.His attacks are easy to block aside from the poison breath one. If he throws that at you you may want to heal it with a Tasty Tonic.
Eventually though Gloomtail will start charging an ultimate yellow breath. Well yellow was the wrong color! Still it's a nasty attack. It's very very much recommended that you have Vivian use Veil or be prepared to face 15-16 damage to both of your characters. That's the strongest attack yet that's for sure. Anyway watch out for this fatal moment and this fight will work outjust fine.