What is the strategy to beat Dragonite?

I have had great luck with Clefable. Fairy type, for whatever reason, is super effective against Dragon types. Clefables are beefcakes, and they can put out a surprising amount of damage. Alternatively a decently powerful Jolteon will do just fine.

Remember, you only get a 20% bonus either way for typing. Focus on building a diverse, high CP team and you'll do fine.

The strategy behind defeating Pokemons lies in the "Type" of Pokemons; by using a Pokemon with a super effective move. Dragonite is a dragon type Pokemon, which is weak against Dragon-type & Fairy-type moves (Fairy types come in later generation of Pokemon series).

So, 1 strategy is to use a Pokemon with Fairy-type moves against Dragonite to defeat it.

There are many Pokemons in Pokemon GO that may know Fairy type moves (refer to the wiki linked above) and you can use those moves to effectively defeat Dragonite easily.

An alternative solution is to look at the other type of Dragonite and which is - Flying.

Flying type Pokemons are weak against - Ice, Rock and Electric.

But here, Electric won't work well as Dragonite has high Electric moves resistivity.

So, the key to defeating Dragonite is to use a Pokemon with Rock or Ice type moves in Pokemon GO.

Here are the possible Rock and Ice type Pokemons you can use against Dragonite (if they have ice or rock type move):

1. Dewgong - An Ice type Pokemon which is sure to have either a Basic or a Special take of Ice type. I've again and again used Dewgong's Ice type super-effective moves to defeat Dragonite.
2. Slowbro - Again, Slowbro would sometimes have Ice type move like Ice beam which is a super effective move.
3. Golem - A Pokemon with rock type moves
4. Aerodactyl - Aerodactyl is Rock and Flying type Pokemon which may have those super-effective rock moves against Dragonite.

There are many other Pokemons like Diglett, Shellder, Kabutops, Lapras, etc which may have Ice or Rock type move which can be used to defeat Dragonite faster as these moves are super-effective to it.

The trouble with using Clefable against dragonite is that Clefable's normal moves are not fairy at all! They are "Pound" (normal) and "Zen Headbutt" (psychic). Neither is specially effective against dragons. The best Clefable charge move against dragons is "Moonblast", but you don't use it often enough to defeat Dragonite. Dragons with Steel Wing are super effective against Clefable. Wigglytuff fares just as poorly with similar move disadvantages. You have to forget about the so-called fairy advantage in Pokemon Go.

What I have succeeded with repeatedly is big Water pokemon: Vaporeon and Gyarados. Sunday I trained at a friendly Gymn with great prestige and with the lower three Pokémon being 2250 Arcanine, 2250 Charizard, and 2400 Dragoniite. One Vaporeon defeated Arcanine and Charizard then weakened Draginite enough so that the second Vaporeon defeated not just Dragonite but also the Snorlax above. Vaporeon followed by Gyarados was equally effective, but Vaporeon followed by Clefable was never able to defeat the bottom three. The Vaporeon have Water Gun/Hydropump, the Gyarados has Bite/Dragon Pulse, and Clefable Pound and Moonblast. My best guess is that the Dragonite had Steel Wing/Dragon Pulse.

I use my 1400 dewgong to defeat many 2300+ dragonites. The frost breath is super effective against the dragonite and it can shoot a frost breath each second which makes it more lethal.

I find that my 1700 CP Steelix can take down a 3200 CP Dragonite as long as I dodge Hyper Beam! Steelix is a Steel-type so Hyper Beam is less effective against me. My primary move is Dragon Tail, which is super-effective against Dragon-types and I have a Steel-type charge attack. I bring out Steelix for every Dragon-type. That said, Cloyster and Dewgong with Ice-type moves also work.