For quite some time, I've been thinking about a Roserade Castform team for double battles, but I'm not that familiar with competitive battling to know how well they'd fair. The idea was sort of like this:

Roserade
Timid

Sunny Day
Rain Dance
Sludge Bomb
(Not sure)

Castform
Modest

Thunder
Solarbeam
Weather Ball
Ice Beam


How well would this work?

Okay, after sleeping on it, I've thought about a better setup.

Instead of Sludge Bomb, Roserade gets Shadow Ball and Extrasensory.

The basic idea behind this team is for Roserade to use the appropriate weather move for any incoming attacks (Sunny Day to counter Ice attacks, Rain Dance to counter Fire)

Then, Castform hits them with the appropriate move. Basic type matchups.

What I like is the potential to counter any STAB with a type advantage.

If there's a nasty looking fighting type, use Extrasensory. Psychic, Shadow Ball. Ice gets weakened with Sunny Day, then hammered with Weather Ball. Fire gets weakened by Rain Dance, followed by Weather Ball. Against Flying, depending on the secondary, I can use Ice Beam, or Rain Dance/Thunder.

I made Roserade Timid, and Castform Modest, just to make sure that the weather can come down before the boosted attack. This set can also counter most weather-based strategies, either by laying down the polar opposite weather condition immediately after, or, if their weather will come between Roserade and Castform, using the correct attack, with Roserade's coming the next turn.

Sunny day reduces water, not ice.

Castform has rather low stats regardless of type so you're better off with him setting up the weather and having roserade use weather ball.

Castform
Impish
Leftovers
252HP 252def 4spd
-Sunny day
-Rain dance/Hail
-Thunder
-Fire blast/Blizzard

Roserade
Black sludge/ salac berry
Timid @ Natural cure
252S.atk 252speed 4def
-Weather ball
-Sleep powder
-Solarbeam/Grass knot
-Extrasensory/Shadow ball/Sludge bomb

I'm sorry, I didn't realize.

I just remember my Swampert's ice beam losing strength during strong sunlight. But that just could have been his low special attack.

I came up with the idea when I noticed that most of Castform's moves relied on him taking up slots to cast the appropriate weather move. This means that he has to waste a turn before he can attack. But if he's in a double battle, some of those moves can be cast by the ally, and providing they move faster, this would allow him to land his weather-enhanced attacks on the first turn.

This will be much easier for you in Gen V when you can use Politoed or Ninetales to set up weather (Looking forward to that.)