Cloyster

My 1500CP Dewgong has WRECKED Dragonites with the level CP

If you dodge correctly it will easily take it out, and I do mean easily

I can finish a fight with Dewgong with more or less half health left

Try it, I think you'll be pleasantly surprised

It depends on how good you are at dodging. I have a 982 Cloyster that I regularly use to train against a 2097 Dragonite, and as long as I dodge everything, I can take it out consistently. Assuming your 1800 Cloyster really has Frost Breath / Blizzard, you should have no trouble taking out 3000+ Dragonites. You just need to get the dodge rhythm down: 3 or 4 quick attacks, then dodge, and repeat. Only use Blizzard after you dodge a charge move to reduce the risk you'll take a full Dragon Claw during the animation.
Plus, powering up a pokemon past 30 costs way more than it's worth. :(

Dragonite's double weakness to ice (flying/dragon) mean ice types wreck it even with lower CP. Lapras is ideal but Cloyster will do just fine.

It wrecks Dragonites, I've got a nice Cloyster as well I look forward to powering up someday, but Dragonites are rarely seen in gyms in my area so it hasn't been a priority for me. If you consistently see Dragonites in gyms in your area, that fella will be very useful!

Yes it is. A well trained Cloyster can beat a Dragonite with just about half health if your skill is good.

I would say no, but that's because I don't want to spend 3/4 of a battle dodging attacks. If you don't mind doing that, I'd still say wait: if you have a Slowbro or a Blastoise or something else more useful than a Cloyster with ice attacks, I'd power up one of those first. My thinking is that Cloyster is only really useful against Dragonites (and with such a low max CP, you'd never want to use it as a defender), but a confusion/ice beam Slowbro is a pretty good all-around defender in addition to being able to wreck high CP Dragonites.

MY ~1500ish Frost Breath/Blizzard Cloysters (I have like 5 of them) easily destroy Dragonites with 2500+CP. Just make sure to dodge their Dragon Pulse/Hyper beam and you are good to go.

Your cloyster will do some amazing damage vs a dragonite. Put it to work. You don't even need to power it up. Ice really makes those dragons fall.

Any Pokémon's with Ice Skill will do good to Dragonites still you will be amazed if you came to visit my place as we have 1 level 10 gym that is being defended by all 10 Dragonites with the lowest CP of 2800++. I just hope the attackers would have atleast more than 5 Pokémon's with Ice Skills. HAHA...