Froslass is a Pokemon I have had before. It was very useful against Cynthias Garchomp due to its Blizzard attack. (KO in one hit).Weavile is very fast, and great attack. But Froslass's sp.attack is useful as well because it can learn blizzard, shadow ball, Psychic and Thunder.destiny bond is also a GREAT help against dark types. Weaviles move pool is smaller, but I would still pick weavile for speed and an amazing sweeper.
Long story short, Weavile if you need sweeper, and froslass for wall.
Hope I helped and sorry it was so long.