Cloyster double ice moveset

Have made 5 attempts for an Avalanche Cloyster and have 3 HP and 2 Aur beam.

Blizzard is a good move and only a little worse than Avalanche. I have that movesett on a level 30 Cloyster I powered up to go through 2 Dragonites with proper dodging back before gen 2

Yes. IS is a little slower, but in the absence of a good FB user, it will do. Blizzard is still able to get it done, despite the availability of avalanche.

I have a Cloyster with those moves at level 30 from before Avalanche was an option. It's still pretty awesome. It's my go to for Dragonites. I just got one with avalanche, but haven't had much chance to try it out yet.

Yep, just do it. It was a time I didn't have any good IV Lapras but whole bunch o'Cloys and they did well.
The only thing I don't like about them is their low HP. Yesterday I got lucky and evolved a 91 15/15/12 L30 Shelder and it gotten IS/A, but still HP is 84 only. L29 Piloswine 10/15/14 however is 151 HP.

I would say yes I have a 1800+ cp cloyster with Ice shard and avalanche and it goes through 3k dragonites no problem

I'm not sure I would power him up. Use him as a gym prestiger, then hunt for a high level Shellder, even if the IV's aren't so great. Evolve him, and you may get ice shard / avalanche (took me 3 tries). For an attacker, you don't need great IV's.