Just run rain and Nasty Plot, it's what I'm doing currently (though I'm still avoiding Thunder, as I don't want to be caught high and dry at any point). I'd also recommend Psyshock by quite a large margin, as then you can 2HKO Chansey if you need to with a +2 boost. The main reason why I've now put Raichu on my team is that it gets through Marowak which people are inexplicably mad over (it's fine, but not to the extent it's being used in the tier currently) and it can get through Special Walls with Psyshock.
I think Electric Terrain and rain have a really nice overlap - but not from Thunder spam (at least on Raichu that can let you down, it is much better on Koko). It means you have two constant sources of doubling your speed that you can whip out when you please, and if your Pelliper faints or your Koko faints you always have options. It's pretty much a weather offence dream, and means that rain is still above and beyond the best weather out there.