Where can you battle Sina and Dexio post-game in X and Y?

Question

Apparently they're the only strong trainers post-game.

Answer 1

You cant battle Sina and Dexio post-game.
However you CAN battle the E4 (obvious location) , restaurant Le Wow (lumiose city), , Inver (route 18) , and the Battle Maison (Kiloude city) for strong post-game trainers.

Sauce: Google, alot of Google.

Comments

Thanks, I thought it said somewhere you can battle them. I'll do these battles instead. I need high level battles because I want my Heatran to learn Magma Storm.

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Google Sauce? What about BBQ Sauce and Mustard Sauce?

Answer 2

Dexio and Sina are first met by the player in Route 4 where they discuss about Flabébé and the Fairy type. Dexio then introduces himself as a Pokémon-voyage veteran who received his Pokédex from Professor Sycamore two years beforehand. In the gate to Lumiose City, Dexio tells the player about friendship and gives the player TM27 (Return). After arriving at Lumiose City, Dexio tells the player to follow Sina to Professor Sycamore's lab.

>Further on, he and Sina give the player the Coastal Kalos Pokédex at Route 8 and the Mountain dex at Route 13. The duo are met soon after at the Kalos Power Plant, disguised as the Masked Heroes, attempting to stop Team Flare. -Bulbapedia Dexio Page and Bulbapedia Sina Page

It says nothing about them battling them post-game. So you can't battle them post-game. :(

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Source: Above Links

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Hope I helped!!