I didn't look too hard, but I was just looking at the pokemon that can learn Sunny Day, do not share common weaknesses with your other pokemon (preferably resisting some of those weaknesses), and fairly speedy but bulky enough to take a hit or two...
The one I found that kind of fit all those criteria was Thick Fat Miltank, which resists fire and ice which Cherrim is weak to. 100 Base Speed isn't amazing, but pretty much anything faster than that can't take too much of a hit. She has access to Helping Hand, Milk Drink, Heal Bell and Seismic Toss, so for support she is ok. With Chople Berry and very little HP EVs, no Close Combat (or Aura Sphere) OHKOs her, so the only worry (of not being able to set up sun) would be sleep. That said, she doesn't really benefit from Sun at all.
Alternatively, Intimidate Arcanine can be pretty useful (Intimidate in general is awesome in double). Somewhat fast, has some bulk, has Extreme Speed, can take advantage of sun, resists most of Cherrim's weaknesses, but of course the bad part is that it shares the same weaknesses with Rapidash. Intimidate helps with the Rock and Ground weakness a bit as most of those moves (Earthquake, Rockslide, Stone Edge) are physical.