I've been meaning to catch a Castform for a while now, and I also need a Vanillish. Both of them can only be encountered through SOS calls, and both of them require Hail (well, just weather for Castform, but you know...). I managed to hold out long enough in the desert to find a Gabite, but that was at a lower overall level so my Pokemon could hold out longer because they were taking less damage from the wild Pokemon.
I'm basically going to have to put together an Ice type dedicated to surviving long term in battle, equipped with False Swipe, ideally with some defensive bulk and/or health restoring ability. Ideally it would also be able to learn something Super Effective against Ice types, to keep the other slot clear and ensure the surviving Pokemon is calling for help as much as possible.
At this point, it looks like I'm stuck with Sneasel/Weavile. They appear to be the only Ice type Pokemon that can learn False Swipe at all. While they do have access to Metal Claw through level up, as well as both Brick Break and Low Sweep through TM... Aren't they more "glass cannon" Pokemon? They aren't exactly meant to go the distance in the endurance match that is SOS Battles.
Anyone have a recommendation?