This is a pretty solid team, you don't have everything covered perfectly, but you a pretty solid check to just about everything in the metagame, which is pretty great for a stall team.
My only team suggestion honestly is to maybe toy with a speedy-flamethrower Blissey over Zapdos as it would cover a lot of the same threats like Venusaur and Gengar, and still provide a pokemon to roast Scizor, Forretress, and Ferrothorn. At the same time it would outright counter Thundurus-T, Magnezone, Volcarona, and Hydregion, which otherwise you are checking. You also gain a more solid counter to Heatran and Ninetails in the sun while Starmie can barely touch. If you are interested here is the set:
Blissey (F) @ Leftovers
Trait: Natural Cure
EVs: 252 Def / 164 SDef / 92 Spd
Bold Nature
IVs: 0 Atk
- Toxic
- Softboiled
- Flamethrower
- Seismic Toss
Investment in speed creeps on standard 8 speed scizor to roast it before it can attack, and investment in spD as opposed to HP makes Venusaur get less HP back from giga-drains. What you lose from this is the ease to cover super-power Tornadus and a few last minute hax checks you get to defend yourself from various physical pokemon. I don't know if its worth the switch, it is something to toy with though if you are interested.
Now for some nitpicks:
-I highly suggest you give Hippowdon ice fang, this would allow you to counter Garchomp, Salamence, and Dragonite much better without being forced to whirl wind them with Skarmory. I would run ice fang over whirlwind or stealth rock (and then give Celebi stealth rock in maybe the last slash). If you do anything do this, because currently dragons have a good chance to outlast you, that should not be happening.
-I am not sure if someone caught this yet, but your Tyranitar is timid... change that to Jolly asap, also give it 252 speed, not just 248 to speed tie other tyranitars, I am not sure why you have that there.
-I suggest you ditch Skarmory's speed creep on wobbuffet / other skarmories, you have plenty of pokemon on this team that could stop an opposing skarmory if whirl-winded into. Grab that extra defense to help out against fire-punch dragonite and friends.
-I highly suggest you run roar on Zapdos (in the last slot) so you can take a hit from Volcarona, and get it out killing it. With 2 hits of stealth rocks and sand, its dead. It's also is a nice check to your current Volcarona weakness. Also, I don't know why you are running volt switch honestly, this is a stall team, you have like almost no momentum to play with. I highly suggest thunderbolt over volt switch for the power and paralysis hax.
-Your celebi would like psychic, not only for Breloom, for to check Thundurus-T which other wise you can only check with Tyranitar. With heat wave Zapdos, you really don't need HP-fire. Perish song is preferable in the last slot to check Reunicles better with. I would run a set up `.
-I suggest you just boot up Celebi and Zapdos's speeds to 32 to out speed jolly tyranitar and get on that common speed benchmark, the loss of bulk is negligible compared to the advantaged of being on that speed tier.
best of luck