Well. It's 3 Water and 1 Fire for Acid Rain. 2 Water and 1 Fire for Acid Bolt

Atlantis using King of the Deep 3 Water, 2 Random (if he gets one in Fire) 290 can cast rain

Caelum using High Seraph 2 Water , 1 random. Can cast bolt if they get a fire, rain if they add an item.

Tien Chi Base Celestial Master, 1 Fire, 2 Water 1 Random. Possibility to get random in water and cast the rain, or use items, with Spring and Autumn, get 2 Random, better chance to get the third Water.

Not too many nations can do it. I don't think it's quite as common as you'd think.