Should I power up/keep/use?
I have a 97.8 Cloyster with Ice Shard Aurora Beam. It's currently at 1906 CP. Shellders aren't common in my area so without this event, me getting the Cloyster wouldn't have been possible. Anyways, should I use him or wait till the event is over and hope I get enough candies and Avalanche??
Answers
I expect you already know IS/AB is not a good moveset. This site ranks it as the worst non-legacy moveset. At 1906 CP, it's too high CP to be used well as a prestiger. If you don't have anything better, you could use it for attacking Dragonites, but there are many other mons that will perform better.
I'd suggest trying to catch more ice types (and things that evolve to ice types) during the event, which is still on going, and paying attention to how gyms change in the rework, which begins tomorrow.
Snorlax or Blissey will do the job too - a bit slower, but with 1/2... 2/3 health left and no fear for dodge-bug. - Blissey with DG performs good. Gyarados with dragon moves is another option.
I have got a 2100 CP-Cloyster FB/AV, but i hardly use it anymore. I mostly use Dragonite or Blissey.
Edit:
So yeah, there aren't many other mons that will perform better than Cloyster against Dragonite
I named the criteria for dragonite attackers (speed, potion efficiency, percent live left) - what exactly do you mean by "perform better"?