So! Lets get to the answering, shall we?
So, you have Skarmory. Overall, this set looks really annoying and looks like a great staller! Nice! I would throw a Focus Sash on it, just to be safe from those annoying special attackers, and to be almost assured to get at least one hazard in.
Well, now Venusaur comes into the picture. I would actually make this your mega, and not really change much else. As a Mega Evolution, Venusaur gains neutralities to Fire and Ice, making 2 common types not as much of a worry. Because this is a mega, I would most likely go for more of a Toxic stall set. Here...
Ability: Chlorophyll ---> Thick Fat
252 SpD, 252 Def, 4 SpA
-Sludge Bomb/Giga Drain
With this set, you utilize Venusaur to it's full potential. With this new EV spread, your working more on survival then anything. On the turn you mega evolve, it's advised you use Protect to get your mega stats in. Protect can also be used to stall the opponent out. Leech seed is a good tool to use to get more HP, but substitute can minimize the effects of status and you can do the famous SubTect strat, stalling your opponent to death. The choice between Giga and Sludge is a power vs staying power situation.
So, we have ninetails. Ninetails overall is a nice pokemon, and you have a fairly nice set here. I can't think of anything to change. Good going! By the way, I would go with Will-O-Wisp over Confuse ray. More reliable in most situations.
Rotom Wash @Leftovers
252 SpA 252 HP 6 Spe
With leftovers and Pain Split, you will increase the staying power of your Rotom. Thunderbolt and Volt Switch seem a little overdoing it, so I would replace Thunderbolt with Will-O-Wisp to cripple Physical attackers. Other then that, nice!
Without the mega stone, I would drop mega banette in favor of Sableye. Sableye is a wonderful Prankster user, and has a better typing then M-Banette.
Ability Prankster (HA)
252 SpD, 252 HP, 6 Def
- Foul Play/Night shade
Trust me, Sableye is a lot bulkier then it looks. With the EV spread Sableye effectively stomachs most hits thrown towards it, though I can't assure you you will have fun with fairies with this pokemon. Will-O-Wisp turns you from a Special Wall to a Mixed Wall, while Taunt cripples Blissey and Ferrothorn. Recover increases the staying/stalling power of Sableye. Foul play can be used for that one D-Nite that is going to D-Dance or a Swords Dancer, while Night Shade can be used for consistent damage.
DIGGERSBY DOE. All Shofu references aside, Diggersby isn't the pokemon you really want. Trust me, you will love my little friend named Azumarill.
Azumarill @Sitrus Berry
Ability Huge Power
252 Atk, 252 HP, 6 Spd
I think we've learned by the end of this that looks can be deceiving. We just saw a little gem gremlin that can wall a lightning bolt to the face, and now its Azumarills turn. After 1 Belly drum, (which is easy to do with a Sitrus berry, as you will be left with roughly 3/4 of your health, assuming you go first) you can proceed to wreak the enemy team. Aqua Jet, for example, has a chance of OHKOing a Greninja, and is even more powerful then an, say, Infernape Close Combat. Sound terrifying? It is. While most of the time you will be spamming Aqua jet, Play Rough will deal with the occasional Dragon Type and Fighting Type, while Superpower just simply takes out the rest. Trust me when I say this; this is good.
So yeah! I hope you like the team I just made you. Good luck, and happy battling!