Swanna @ Life Orb
Ability: Hydration
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Hurricane
- Roost
- Defog
- Scald
As everyone knows, Mega-Venusaur is horrendous to face with water. Personally having a team built with the same mons as yours, I know even better that venusaur just makes you want to press that x because theirs almost no way to properly kill it without kingdra carrying.
And so, I present to you one of the few possible venusaur checks on water.
Another bonus is defog. While stealth rocks doesn't bother the team too much, hazard stacking can really damage the team because depending SS requires a moderate amount of switching, so priority moves can just end you if you aren't careful. Also, the bug matchup can be quite difficult with sticky webs, so swanna not being affected by it AND taking it out is a huge bonus. This prevents Mega-Heracross or Mega-Pinsir sweeps. Finally, although toxic spikes aren't as common, being able to get rid of them is helpful because when your sweepers are poisoned against a team with great walls, it's a recipe for disaster.
If their was a move to change, roost could be replaced by rain dance to give you another rain source and possibly a zard-y counter (win the speed tie to make it locked into a solarbeam).
Some people might think, why choose swanna when you have access to Starmie who has better special attack and speed and does the same job of hitting venu and getting rid of hazards. Well, my reasoning would be that it doesn't get held back by hazards like sticky webs and it doesn't get OHKO'ed by Venusaur.
Here's some calcs (using the venusaur set from Crimson Torrent's sample team because I'm not too familiar with what people use commonly):
Swanna:
252 SpA Life Orb Swanna Hurricane vs. 248 HP / 0 SpD Mega Venusaur: 307-361 (84.5 - 99.4%) -- guaranteed 2HKO
252+ SpA Mega Venusaur Giga Drain vs. 0 HP / 4 SpD Swanna: 186-220 (63.9 - 75.6%) -- guaranteed 2HKO
Starmie:
252 SpA Life Orb Starmie Psychic vs. 248 HP / 0 SpD Mega Venusaur: 273-322 (75.2 - 88.7%) -- guaranteed 2HKO
252+ SpA Mega Venusaur Giga Drain vs. 0 HP / 4 SpD Starmie: 294-348 (113.5 - 134.3%) -- guaranteed OHKO
Overall, Swanna can be used to remove hazards and handle big threats. Sure the 4x weakness to electric isn't pretty, but SS doesn't have too much trouble with electric.
Hope this is actually understandable, made this late at night right before I head to bed.