Sash Omastar with Shell Smash isn't bad. It doesn't rely on rain as much as a Specs set and actually kills scarfers. As long as you keep the Sash intact you pretty much guarantee a kill vs offense without even considering rain. If you get a free Shell Smash in the rain it's pretty much over already. Not sure whether Specs is better or not, but I wouldn't write off Shell Smash too soon regardless because it might just save you when Politoed goes down and you're out of rain. Being too reliant on Rain being up when building a team is never a good idea.
As for teammates for Omastar, Magnezone helps just for Ferrothorn. I run Sub + Charge Beam myself because Choice sets with HP Fire don't cut it in Rain vs Leech Seed + Protect shenanigans. You usually get another kill after Ferrothorn goes down too because you're at about +3 and behind a Sub most of the time. Landorus-T is good for taking on Ground/Fighting/Electric moves and is usually a good pick for Rain teams in general. Manaphy is like mandatory on Rain. Tail Glow + Rest + Hydration is too good to pass up and takes on Chansey like no other mon does.
I've used Talonflame on Rain too and it's not bad like you'd think. Flare Blitz still maims Ferrothorn in Rain and you use it for setup fodder with SD. It also threatens Mega Venusaur who is problematic. I only stopped using it because it's weak to SR (which Ferrothorn unfortunately always carries) and hazard support is hard to fit on fast-paced rain teams.
As for teammates for Omastar, Magnezone helps just for Ferrothorn. I run Sub + Charge Beam myself because Choice sets with HP Fire don't cut it in Rain vs Leech Seed + Protect shenanigans. You usually get another kill after Ferrothorn goes down too because you're at about +3 and behind a Sub most of the time. Landorus-T is good for taking on Ground/Fighting/Electric moves and is usually a good pick for Rain teams in general. Manaphy is like mandatory on Rain. Tail Glow + Rest + Hydration is too good to pass up and takes on Chansey like no other mon does.
I've used Talonflame on Rain too and it's not bad like you'd think. Flare Blitz still maims Ferrothorn in Rain and you use it for setup fodder with SD. It also threatens Mega Venusaur who is problematic. I only stopped using it because it's weak to SR (which Ferrothorn unfortunately always carries) and hazard support is hard to fit on fast-paced rain teams.