Hi Etherion! While you have a good framework going here, I'd like to make a few suggestions to you.
Unfortunately, your team has massive trouble with Mega Venusaur. In order to rectify that, I would
replace Arcanine with a specially defensive Talonflame (Leftovers, 248 HP, 244 SpD 16 Speed, Careful nature with Brave Bird/Will-O-Wisp/Taunt/Roost). If you'd rather, you can use an offensive variant of the same set with a bit more Attack and Speed investment along with Sky Plate for the item. Talonflame will also help you to take on the Serperiors that gimmepie mentioned in his post. I would personally run the fully specially defensive variant because it allows you to beat Mega Charizard Y without any trouble, which would steamroll your team otherwise.
Next, I would
make Gastrodon specially defensive and run Ice Beam over Earth power. This will help you to beat Thundurus, Mega Manectric, and Mega Diancie. I would run a spread of 248 HP, 252 SpD, 8 Speed with a Calm nature). Earth Power doesn't target anything that Gastrodon would otherwise struggle with in OU save for the rare Toxicroak which you beat with Skarmory anyway. You'll run into dragons and opposing Mega Sceptiles far more often, in which case Ice Beam would be quite useful.
Blissey isn't all that effective in today's metagame, but Azumarill seems necessary owing to the fact that without it, your team would have massive issues with Hydreigon, Goodra, etc. In order to rectify this, I would
replace Blissey with Clefable and replace Azumarill with Tyranitar. Clefable acts much like Blissey, only it is more reliable and takes on pesky Dragon-types. There are two ways you could go with your Clefable. The first is a bulker variant with Leftovers, a Calm nature, and a spread of 248 HP/172 Def/84 SpD/4 Speed with a set of Moonblast/Flamethrower/Softboiled, and Calm Mind. This Clefable is an extremely effective late-game sweeper and mid-game sponge. This spread allows you to avoid getting 2HKOd by Mega Lopunny's Return and Thundurus-I's Thunderbolt, among other things. Since you already have a special tank in Gastrodon, you might want to consider running a Clefable with 248 HP/172 Def/84 SpA/4 Speed with a Bold Nature and a Life Orb. This allows Clefable to 2HKO Chansey the vast majority of the time at +6, which helps you to beat stall teams easier. Both sets are equally effective, but if I were you, I would run the latter over the former.
Finally, with Blissey gone, you need a Stealth Rocker. Tyranitar fits the bill in this situation as it acts as a Talonflame/Mega Charizard X check for your team which you desperately need. If I were you, I would run a Relaxed-natured Tyranitar with 248 HP/80 Def/176 SpD/4 Speed with Stone Edge/Thunder Wave/Stealth Rock/Pursuit and a Chople Berry. The Chople Berry will allow you to take 1 Focus Blast from Gengar, Mega Charizard Y, or Alakazam, which allows you to retaliate with a Thunder Wave, essentially neutralizing the threat (Your first line of defense for Gengar and Zard Y should be Gastrodon and Talonflame, respectively, but the Chople Berry helps as a last line of defense against these threats. If you want, you can run a Shuca Berry variant instead, which would help you take on Zard X even more effectively.
Other than some EV nitpicks, those are all the edits I have. As I see it now, the biggest threat to your team is Heatran, but you can surprise it with an Earthquake from Mega Sceptile, and you can stall it out with Talonflame as long as you don't throw Talonflame in on a Toxic. I have put the edited team in a pastebin below so you can easily import it to Showdown. I hope I was helpful!
PASTEBIN:
https://pastebin.com/hUeTdivS
