Zygarde, the Kalos region’s third Legendary Pokémon, resides at the end of Resolution Cave on Poni Island. If you caught it, you should pay a visit to the Aether Foundation trailer at Route 16, east from the Pokémon Center. Inside, you’ll meet Sina and Dexio again. They’ll offer to unlock Zygarde’s true power for you, but only after testing you.
Pokémon Trainer Dexio
For the most part, Dexio’s team is largely unchanged since you fought him in front of Poni Meadow. Espeon is a speedy special attacker that can use Psychic for STAB, Shadow Ball to hit Ghosts, and Dazzling Gleam to counter Dark-types. Steel-types are advised, unless you’re confident.
The Alolan Raichu is also a speedy special attacker, but has slightly lower damage output than Espeon. It can use Psychic and Thunderbolt for STAB, and Light Screen and Reflect to bolster its party’s defenses. Ground or Dark-types are strongly recommended; Bugs and Ghosts should also do the trick.
Slowking is a slow-moving, but bulky special attacker. It can use Psychic for STAB, and forgoes Water-type STAB for Power Gem, Flamethrower and Ice Beam. Dark-types are recommended; Electric and Ghost-types are also solid choices. Grass and Bug-types are super-effective, but weak to Slowking’s coverage moves.
Alakazam is fast and hits hard with its special attacks, but incredibly frail. Since Dexio gave you his Alakazite, his Alakazam won’t Mega Evolve. It has Psychic and Psyshock for STAB (each targeting a different defense), and Focus Blast to handle Steel and Dark-types. Ghosts are ideal, but they should have full fealth.
Finally, Dexio wants to show off his Zygarde 10% Forme. This dog-like variant of Zygarde has great Attack and Speed, but poor bulk. For attacks, it has Dragon Pulse and Earthquake for STAB, Crunch to hurt Ghosts etc., and Camouflage to become a Normal-type inside the building. Ice-types are its bane; Fairies are great too.
The Zygarde Cube
Pass the test and you’ll receive Dexio’s Zygarde 10% Forme and a Zygarde Cube containing 40 Zygarde Cells. Now you’ve got all the tools to make the ultimate Zygarde! To do so, inspect the Reassembly Unit towards the right. Make sure you have the Zygarde from Resolution Cave with you.
Not including the instructions, two options will be available: Assembly and Separation. The first option–Assembly–lets you build a Zygarde using the Zygarde Cells currently in the Zygarde Cube. Meanwhile the second–Separation–lets you break up a Zygarde into its constituent Zygarde Cells, and transfer them to the Zygarde Cube.
To proper understand everything, you need to know there are exactly three Zygarde forms, each comprising of a specific amount of Zygarde Cells. First you have Zygarde 10% Forme, which is the dog; this form is built from 10 Zygarde Cells, hence the 10%. This is obviously the weakest form, but still useful because it’s the fastest one.
Next, you have Zygarde 50% Forme, which is the serpent–and also the “original” Zygarde introduced in Pokémon X and Y. This Zygarde is built from, you guessed it, 50 Zygarde Cells. In this guise, Zygarde is roughly at half power. Finally, there’s Zygarde Complete Forme, which resembles a humanoid… or a mecha, if you prefer.
This final form is built from 100 Zygarde Cells and is Zygarde’s true, ultimate form. However there’s a slight complication: Zygarde Complete Forme is a temporary form, not a permanent one. Instead, what you normally have is a Zygarde 10% Forme or a Zygarde 50% Forme that has the Power Construct Ability.
During battle, a Zygarde with the Power Construct Ability automatically transforms into its Complete Forme after going below half HP. This means you have to start off with the weaker Zygarde and hope that the foe lowers your HP just the right amount. But if it works, it’s worth it since Zygarde Complete Forme is pretty hard to defeat.
Anyway, that’s how Zygarde works, but first we need to properly build it. If you haven’t already noticed, the game has kindly provided exactly 100 Zygarde Cells for you to work with: 50 from the Zygarde in Resolution Cave, 10 from the Zygarde obtained from Dexio, and 40 from the Zygarde Cube.
What you need to do is find a way to properly combine these cells together. First, choose one of the Zygarde you own and select the Separation option to convert it into Zygarde Cells. Or both of them if you’re not feeling particularly attached. The result that you want is a single Zygarde or no Zygarde at all.
Next, choose the Assembly option. If you kept a Zygarde, choose “Using Zygarde and the Zygarde Cube”. Then choose the Zygarde. If you did it right, your chosen Zygarde will gain the Power Construct Ability. Or if you separated both Zygarde, choose “Using the Zygarde Cube only” to generate a brand new Zygarde with Power Construct.
Last but not least, the Zygarde Cube has two additional functions. If you use it from your bag, you can teach various moves to any of your Zygarde, including its three signature moves (because one isn’t enough). Also, if you have a Zygarde with Power Construct, you can switch between its 10% and 50% forms.