Before all of this, make sure you have Girafarig in your first party slot, and Dunsparce in your last! This will open the first chamber door. There is a puzzle in the first chamber, which the answer is in the pattern of jars in the cabin at the frozen lake outside of Frostveil City on Route 15. The order is the following:
Red, Green, Red, Blue, Blue, Yellow, Green. Remember this order. (or simply keep this wiki page open, you lil' devil you.)
Regice can be located in a large (closed) stone door along Route 15 through an icy, large arch. To unlock, head into Frostveil and go right of the gym (or left, if you're facing the gym,) and find the broken portion of the stone fence. There is a Rock Climb path. Approach it, climb down, and there is a cave. Upon entering and approaching a braille-coated door, upon interaction it will read "There are strange markings on this wall. What could they mean?". This is where Girafarig and Dunsparce come into play. They will unlock the door by having them only in the order as listed at the top, with Girafarig first and Dunsparce last.
Once you've completed the first chamber, the second door requires Rock Smash to open the cracked stone door.
The second puzzle requires that each dot has been connected to its respectful color tile. The floor will rumble, and it will say doors have opened far away. Head back down Route 15, and pass through the ice archway and the door that was closed there will now be open. Regice will be down the tunnel.
6 Defence ev'sw