Okay... so this set may be a bit unorthodox, but take a look...
Gyarados @ Leftovers
Adamant, Intimidate, 252 Atk / 8 Def / 248 Spd
~ Substitute
~ Waterfall
~ Stone Edge
~ Toxic
Now, I know what you're thinking--no Stealth Rock? Blasphemous! How can we run a team with Heatran's phazing capabilities and against Kyurem-B, no less? Well, let me break this down for you and explain why I think that this set would absolutely wreak the opposing team's best plays against our own.
We've been talking about Mamoswine and Heracross, wishing that we could come up with a counter to both of them. Look no further than Gyarados, folks. Even
after switching into Stealth Rock, Gyarados can survive two hits from either of Heracross' STAB attacks, Mamoswine's Icicle Crash, and obviously doesn't care about Earthquake thanks to Intimidate and its many resistances. The only thing it minds from Heracross is Stone Edge (not the best attack to lock itself into), and it simply doesn't care about Mamoswine. With the given EV spread, it also out-speeds Team 1's Mamoswine, meaning that it can't ever switch-in to bypass Intimidate, as it will be swiftly KO'd by Waterfall.
More importantly, Team 1 really only has one current switch-in to Gyarados, and that's Rotom-W. Heracross surely won't switch-in, and neither will Mamoswine. Kyurem-B won't want to switch in fearing Stone Edge, and we out-speed it, anyway. Landorus is faster than Gyarados, and Psychic will definitely take a chunk of health... but Waterfall is a swift KO. But, with Toxic, Rotom-W will not want to come in against the almighty Magikarp evolution. When Gyarados comes in against something from Team 1, it can set up Substitute on the switch to Rotom-W. Since Gyarados comfortable out-speeds it, it can quickly land a Toxic against it, meaning that it's days will be numbered. Team 1 relies on a very healthy Rotom-W to protect against Heatran and Keldeo. With its primary counter being inflicted with toxin and with no way to protect itself from dying other than Pain Split, Team 2 will immediately steal momentum.
Even more importantly, when Rotom-W Volt Turns out (after being inflicted with Toxic) and breaks Gyarados' substitute, what will it switch to? Kyurem-B is out-sped and takes decent damage from Stone Edge and would COMPLETELY die from Toxic. Landorus can come in and go for a Psychic, but Gyarados will have at least 67%-73% health by this point (due to Leftovers recovery even after Stealth Rock damage / Substitute) and can live a hit to KO it with Waterfall. Heracross can come in and go for a Stone Edge, but we can take advantage of the locked attack and bring in Keldeo. Mamoswine can't come in at all. But Rotom-W has no choice but to Volt Switch, as Hydro Pump does pitiful damage, and Thunder Wave and Pain Split can't pierce a Substitute.
Let's take a look at some calculations:
Heracross:
-1 252 Atk Heracross Close Combat vs. 0 HP / 8 Def Gyarados: 76-90 (22.96 - 27.19%) --
possible 5HKO
252 Atk Heracross Close Combat vs. 0 HP / 8 Def Gyarados: 114-135 (34.44 - 40.78%) --
64.18% chance to 3HKO
-1 252 Atk Heracross Megahorn vs. 0 HP / 8 Def Gyarados: 76-90 (22.96 - 27.19%) --
possible 5HKO
252 Atk Heracross Megahorn vs. 0 HP / 8 Def Gyarados: 114-135 (34.44 - 40.78%) --
64.18% chance to 3HKO
-1 252 Atk Heracross Night Slash vs. 0 HP / 8 Def Gyarados: 60-71 (18.12 - 21.45%) --
possible 7HKO
252 Atk Heracross Night Slash vs. 0 HP / 8 Def Gyarados: 90-106 (27.19 - 32.02%) --
possible 4HKO
-1 252 Atk Heracross Stone Edge vs. 0 HP / 8 Def Gyarados: 170-202 (51.35 - 61.02%) --
91.8% chance to 2HKO
252 Atk Heracross Stone Edge vs. 0 HP / 8 Def Gyarados: 256-302 (77.34 - 91.23%) --
guaranteed 2HKO
Meanwhile...
252+ Atk Gyarados Waterfall vs. 0 HP / 0 Def Heracross: 178-210 (59.13 - 69.76%) --
guaranteed 2HKO
In other words, Gyarados completely laughs at every attack except for Stone Edge and can threaten an immediate gyaranteed 2HK0, even without Stealth Rock support.
Mamoswine:
-1 252+ Atk Life Orb Mamoswine Icicle Crash vs. 0 HP / 8 Def Gyarados: 160-188 (48.33 - 56.79%) --
43.36% chance to 2HKO
252+ Atk Gyarados Waterfall vs. 0 HP / 4 Def Mamoswine: 336-396 (93.07 - 109.69%) --
56.25% chance to OHKO
Meaning that Gyarados can switch in to any attack that Mamoswine throws at it, survive with around 25% health even after Stealth Rock damage, and possibly KO with Waterfall. Waterfall is also an outright KO after Life Orb damage! It can also switch into Earthquake or Ice Shard (or Stealth Rock) without worry. In fact, Gyarados likely won't even need to worry about Stealth Rock, as it can switch INTO the move, avoid it, and threaten Mamoswine with Waterfall or Substitute / Toxic on the switch. Did I mention that Team 1 doesn't have anything to switch into Toxic besides Heracross, who won't want to risk switching into Gyarados anyway?
Also, food for thought:
252+ Atk Gyarados Aqua Tail vs. 0 HP / 4 Def Mamoswine: 374-444 (103.6 - 122.99%) --
guaranteed OHKO
If enough people like the guaranteed OHKO on Mamoswine with Aqua Tail over Waterfall, I'm definitely fine to try it. The extra power is pretty awesome here.
Kyurem-B:
-1 0 Atk Kyurem-B Dragon Tail vs. 0 HP / 8 Def Gyarados: 84-99 (25.37 - 29.9%) --
possible 4HKO
0 Atk Kyurem-B Dragon Tail vs. 0 HP / 8 Def Gyarados: 124-147 (37.46 - 44.41%) --
guaranteed 3HKO
252+ Atk Gyarados Stone Edge vs. 252 HP / 64 Def Kyurem-B: 218-258 (48.01 - 56.82%) --
89.06% chance to 2HKO
252+ Atk Gyarados Stone Edge vs. +1 252 HP / 64 Def Kyurem-B: 146-174 (32.15 - 38.32%) --
97.75% chance to 3HKO
Even after Kyurem-B uses Bulk Up (+1 to defense, +0 to attack due to Intimidate), Gyarados beats it one on one and can still break its substitute. It also out-speeds Kyurem-B, meaning that it can't PP stall with Roost and Substitute. More importantly, a swift Toxic from Gyarados will end any sweeping Kyurem-B can hope to achieve. Kyurem-B also can't switch in, risking a near 90% probable 2HK0. Worst case scenario, we still have Heatran. And Gyarados isn't dead weight when we need to break its substitute.
Landorus
252 SpA Life Orb Sheer Force Landorus Psychic vs. 0 HP / 0 SpD Gyarados: 153-181 (46.22 - 54.68%) --
8.2% chance to 2HKO
252 SpA Life Orb Sheer Force Landorus Psychic vs. 0 HP / 0 SpD Gyarados: 153-181 (46.22 - 54.68%) --
guaranteed 2HKO after Stealth Rock
252 SpA Life Orb Landorus Hidden Power Ice vs. 0 HP / 0 SpD Gyarados: 91-108 (27.49 - 32.62%) --
possible 4HKO
252 SpA Life Orb Landorus Hidden Power Ice vs. 0 HP / 0 SpD Gyarados: 91-108 (27.49 - 32.62%) --
99.32% chance to 3HKO after Stealth Rock
Gyarados can switch in on Rock Polish or Earth Power, taking no damage. It can also take Stealth Rock, Hidden Power Ice, AND a Psychic, and still potentially survive even with maximum damage after Leftovers recovery. More importantly...
252+ Atk Gyarados Waterfall vs. 64 HP / 0 Def Landorus: 306-362 (91.34 - 108.05%) --
50% chance to OHKO
Gyaranteed OHKO after Landorus takes Life Orb damage only once. Or, again, if we wish to secure the OHKO 100% of the time...
252+ Atk Gyarados Aqua Tail vs. 64 HP / 0 Def Landorus: 344-408 (102.68 - 121.79%) --
guaranteed OHKO
So, Gyarados does not completely counter Landorus, but it can survive a Psychic and pretty much OHKO back one way or the other. Also, should Gyarados be behind a Substitute on a switch, it's pretty much sealed. Even if Landorus switches in on Stone Edge...
252+ Atk Gyarados Stone Edge vs. 64 HP / 0 Def Landorus: 127-150 (37.91 - 44.77%) --
guaranteed 3HKO
It still takes more damage than it'd like to. Landorus shouldn't be switching into Gyarados, anyway.
And finally... while Rotom-W's Volt Switch destroys Gyarados, we can still Substitute before it or Toxic it on the switch. Gyarados forces out Mamoswine and Heracross, two Pokemon that Team 2 currently fears, and can Substitute on the switch to Rotom-W whenever it wants.
Gyarados allows Team 2 to maintain offensive momentum and is only straight-up walled by one of Team 1's Pokemon, who absolutely DESPISES Toxic, while simultaneously acting as a fantastic counter to Heracross and Mamoswine. It also out-speeds and puts an end to Kyurem-B, and can survive even the strongest hit from Landorus in a pinch during an attempted sweep and OHKO back after a single round of Life Orb damage.
Finally, I'd like to discuss Stealth Rock and why I feel that the strengths of Gyarados outweigh the need for Stealth Rock. Simply put, the reasons that we'd want to have Stealth Rock--wear down Kyurem-B and Rotom-W, punish the multiple switches Keldeo, Hydreigon, and Scizor force, etc.--are accomplished with our team as currently constructed. Toxic kills any hopes Kyurem-B or Rotom-W have of accomplishing anything, whether or not they switch out. And the biggest reason to fear NOT using Stealth Rock is "what if Team 1 picks Volcarana or Dragonite?" Well, Gyarados absolutely, 100% of the time, destroys Volcarana unless it's using Hidden Power Electric, as it can survive any hits and swiftly OHKO with Stone Edge. It can switch in to Fiery Dance and Bug Buzz and take laughable damage. Meanwhile, Dragonite cannot risk Outraging against our team with Heatran and Scizor, nor does it appreciate Intimidate. Stone Edge can take a huge chunk away from Dragonite as well, Celebi can T-Wave it, Gyarados can Toxic it, Hydreigon can out-speed and KO it... we simply don't need to constrain ourselves to ABSOLUTELY using Stealth Rock, simply because most of the good Stealth Rock users do NOT fit out team as well as Gyarados does. I wish we had chosen Stealth Rock sooner, but I'd rather not have it at all and continue to put enormous pressure on Team 1 rather than pick a SR user who will likely end up dead weight on our team. So...
Gyarados for Team 2!!
Feel free to comment on Waterfall vs. Aqua Tail, as Aqua Tail secures OHKOs on both Mamoswine and Landorus, while Waterfall requires that they both take a round of Life Orb damage.