Well for one, Slowbro can't switch into a +2 Houndoom running around. But as far as that goes, Rotom-W appears powerful being able to deal with most of his counters. Amoonguss pairs well with Rotom-W and completely wrecks Mega Bro. I should probably add another pokemon to the defensive core because it gets wrecked by Latios. Hmm perhaps Mandibuzz?That's why I chose rotom wash as he can cripple them all with w-wisp and take them out except for mega slowbro. Your going to need a pokemon that can toxic stall. Gliscor cannot do it because he is a ground type and physically defensive not special, so stalling with sylveon's toxic will be your best bet. If you have a chance you can then go into rotom again then try to see if a volt switch can take it out then.