In Lumiose City, go inside Lysandre Cafe in Magenta Plaza. There's nothing immediately sketchy about Lysandre Cafe other than the music. Go talk to the waiter and he'll battle you! It turns out he's a Team Flare Grunt. After beating him, talk to the waitress to battle her too.

Press "A" against the bookshelf behind the waitress to open the hidden door. Go inside. You'll battle Team Flare Lysandre. He uses a Lv. 45 Mienfoo, Lv. 49 Gyarados, Lv. 47 Pyroar, and Lv. 45 Murkrow. Lysandre will leave after the battle. Go to your left to start going through the maze.

Fight the first Grunt, then take the teleportation tile. Battle Team Flare Aliana, who'll then let you through. Take the green tile on the left. Go north, fight the Grunt, and go into the room to get Revives from the vigilantes you met earlier. Take the tile on the right. Head west into the room. The Grunt inside will give you TM12 Taunt. Pick up the Revive. You can also heal your Pokemon by sleeping in a bed. Go back outside and head east. Take the tile pointing right. You'll be spun around for quite a while. Continue following the only available tile.

Go into the room and you'll fight two more Team Flare members. Afterwards, they'll reveal they're stealing electricity to power the ultimate weapon. Go back outside, take the first tile from the left, go southwest, and take the green teleportation tile. Pick up the Black Glasses. Head north, battle the Grunt, take the tile, and go into the room where you'll find a Rare Candy. Head back to take the tile to end up where you started.

Take the rightmost tile. Take the teleportation tile to pick up a Hyper Potion. Go back, take the tile on the right, fight the Grunt, and take the teleportation tile. Head east, fight the Grunt, and head all the way east through the door. In this room, you'll battle Team Flare Mable, who'll give you the Elevator Key you're seeking.

Go back to the entrance and take the elevator to B2. Head all the way down the hall and you'll find the freakishly tall man you saw earlier with Lysandre. The tall man tells his touching story in a cut scene. Lysandre reveals the tall man's name is AZ and leaves.

Head back to the elevator and go to B3. Here, you'll battle Team Flare Xerosic. After defeating him, pick either the red or blue button. No matter what you press, Xerosic will activate the ultimate weapon! Another cut scene will play, showing the activation of the ultimate weapon.

Now get out of here and fly to Geosenge Town.

Team Flare Secret HQ

Head northwest in Geosenge Town, past the tipped-over cabins, where you'll battle a Team Flare Grunt. Continue northwest after that. Serena will catch up to you. Head into the Team Flare Secret HQ entrance.

Inside the elevator, go up to the controller and press "A", which will take you downstairs. Get out and go all the way down the hall, where you'll find Lysandre. He's trapped a legendary Pokemon to power the ultimate weapon. You must stop him! He'll battle you. Afterwards, take the door to the left and follow the path. You'll battle some more Grunts along the way. Halfway through, Shauna will join in with you and Serena. At the end of the hall, you'll arrive at the doors where the legendary Pokemon is trapped behind.

Inside, you'll first have to battle four Team Flare Admins. Afterwards, they'll run off. Before you take another step forward, save your game. You're about to awaken the legendary Pokemon who you have only one chance to catch!

Walk up to Xerneas (if you play Pokemon X, or Yveltal, if you play Pokemon Y) to wake it up and it'll battle you. To get past this, you'll either have to make it faint or capture it. I recommend capturing it. It takes some patience to catch legendary Pokemon, who are quite a bit harder to catch than your average Pokemon. You want to get its HP very low and keep chucking Ultra Balls at it. Having it asleep or paralyzed also helps. Or, you can take the easy way out by catching it with your only Master Ball. If you mess up, just restart the game.

Right afterwards, Lysandre will come by in a weird outfit and battle you one last time. In this battle, Lysandre will Mega Evolve his Lv. 53 Gyarados into a Mega Gyarados. After defeating Lysandre, the ultimate weapon will get destroyed, and you'll all end up outside.

Now, head back to Anistar City and head south onto Route 18.

Route 18

Terminus Cave

Upon entering Terminus Cave, smash the rock on the left to pick up a Soft Sand. Smash the rock directly north of that to pick up a Max Ether. Smash the rock to the right of the cart for a Revive. Head east and go downstairs. Head southwest. Pick up the hidden Dusk Ball on the rock after hopping over the two ledges. Go north. Right before the bridge, smash the rock to pick up a Star Piece. Go across the bridge and head southwest. Smash the rock blocking the road to pick up a Heart Scale. Pick up the Heat Rock. Go back near the first bridge and head east onto the second bridge.

Hop over the ledges. Go west for an Escape Rope. Head back to the other side of the second bridge and go north. Smash the rock and go inside the narrow passage to pick up a hidden Moon Stone. Head downstairs. Head north past the trainer. Smash the rock and head all the way west for a Dusk Stone. You can also find a hidden Iron on a rock along the way. Go back and head north. Battle the trainer. Head northwest to pick up an X Attack. Then, head east.

Head up the ramp going west. Smash the rocks, battle the trainer, slide down the ramp on the east side, and pick up the Elixir. Head east and you can find a Full Heal. Back to where you were, head south. Go west, smash the rock for a Heart Scale, and pick up TM30 Shadow Ball by the cart. Continue south from the fork past the two trainers. Go west for an Iron Plate. Head east, which leads upstairs.

After going up the first ramp, smash the rock for a hidden Max Ether. Continue north and battle the trainer. A purple-haired guy is blocking the path, telling you you can't go on unless you're at least as strong as the Champion. Oh well; come back here later to catch the legendary Pokemon Zygarde. Smash the rock near him for another Max Ether. Head east for a Reaper Cloth. There's nothing more to do here. Get out of the cave using an Escape Rope.

Outside, follow the tracks to enter the other entrance of Terminus Cave to pick up TM31 Brick Break. Right outside is a Max Ether. Go down to the bottom entrance for a Dragon Scale.

Now, proceed to Couriway Town!