The best way to win the weather war in my experience is to play conservatively with your own weather starter until you've eliminated your opponents. If you're planning on using Politoed, that means don't just recklessly switch in as soon the weather changes. Instead, whittle down their weather starter, or trap it, or whatever you need to do. Sand teams in particular tend to only rely on Tyranitar or Hippowdon to keep their desired weather up, and once that's gone, then their team will quickly be overwhelmed by yours. Additionally, it's never a bad idea to throw Rain Dance onto one or two of your Pokemon just in case something does go awry, and Politoed faints before their weather starter, and even if it's not fainted, you can use the rain to your advantage to force switchs, and rack up residual damage from entry hazards.
[21:32] <@Kaphotics> Found trash bytes from evo chain Roggenrola, Gigalithe.
[21:32] <@Kaphotics> >>Found trash bytes from Gigalithe.
[21:32] <@Kaphotics> wut