Rotom-Wash is extremely good on a rain team.
A good moveset for Rotom-W in rain is
Fourth move of choice, I like Pain Split but there are some other options.
Hydro Pump is STAB and gets a wicked boost from rain.
A nice EV spread would be,
252 SAtk / 252 Spe / 4 SDef or 4 HP
Strategy is tough because it depends on your opponents team and your team.
Hope this helps!!