Question

What level should I evolve sneasel in diamond?

Answer 1

Depends if you want Ice Shard.

If so, level 49.

If not, any time before that. Sneasel doesn't learn anything else that's more useful than Weavile does.

Gen IV Sneasel Moveset
Gen IV Weavile Moveset

Answer 2

I'd wait until it gets ice shard before evolving, which is at LV.49. If you can get one to learn ice punch by breeding, though, any level is good. It also learns agility at LV.20. If you don't want any of these moves, then really you can evolve it whenever you want and it'll come out o.k.