Cool team. Looking at it, I thought opposing chlorophyll sweepers such as Venusaur were easily the biggest threat to your team, especially if in venusaur's case it carries Earthquake to dispose of nintales and heatran. Mamoswine also looks fairly problematic, as it can deal huge amounts of damage to all of your team and can revenge kill both of your sweepers. As KoB said, terrakion can also be difficult for you to handle, as you don't have a safe switch in for it.
First off, I think KoB made a small error in his rate. You should be using Rapid Spin > Roar
(I think he got confused) as this is what your team really needs. Rapid Spin support to help your SR weak weather starter as well as your sweeper salamence. Considering tales will have to switch in a bit it definitely loves rapid spin support that donphan can provide, and rapid spin lets rotom-w volt-switch more, and can free your team of its weakness to spike stacking as well, granted you can get the spin blocker on the other team out of the way. On the subject of donphan, I would bump your attack up just a little bit. 252HP/ 80Atk/ 168Def
gives you enough attack to get the guaranteed 2HKO on landorus-I w/o SR on the field, and also gives your earthquake a nice kick in the guts.
Salamence really wants Fire Blast > Fire Fang
with a naive
nature. Fire Blast muscles through the physical walls that generally give salamence problems, such as gliscor and it allows you to get a guranteed OHKO on skarmory now. Fire Fang has really pitiful power and the targets it hits will be hit harder by fire blast most of the time anyway. Also I would use Earth Power > Earthquake
and Air Baloon > Leftovers
on heatran. Air Balloon lets you take on non superpower mamoswine and scarf landrus once and possibly KO them, while Earth Power does greater damage than earthquake coming off heatrans naturally higher and invested attacking stat.
Now, I agree that the rotom-w set should be changed. I thought that to retain a good way to counter rain teams and beat down a lot of common threats, using Specially Defensive Rotom-W > Scarf Rotom-W
would be a great fit for your team. With its great typing that can take the common attacks found on rain teams, you should have no problems taking them on. Rotom-W can wall the majority of specially orientated sweepers within the tier and cripple them with thunder wave, allowing you to avoid potentially dangerous set up sweepers as well. Pain Split is another cool move, and sapping the health off of opposing weather inducers can help win weather wars. You can still pick up momentum with volt switch, and HP [FIRE] still lets you beat ferrothorn. This set handily beats mamoswine as well, and can cripple terrakion with thunder wave if it tries to switch in
GL with the team