Reshiram

Enter Crystal Peak and follow the singular path forward.

Descend from the ladder.

On this floor, use Surf to follow the path east and then turn north.

Make sure all three crystal switches are enabled.

With that done, go back to the water and climb the east ladder.

Use Rock Climb to scale the wall down before going west.

Use the yellow crystals to scale the wall up and follow the path south.

You’ll see more yellow crystals to your east that take you downward.

Use Rock Climb on them and continue south.

Take the exit you see to your south.

Now outside, scale the wall using the colored crystals.

When you get the choice to climb the blue or red crystals, use Rock Climb on the blue crystals to your east.

Continue climbing until you can enter the mountain again.

Once inside, descend from the platform you’re on using the red crystals to your east.

Then go north and take the first turn west and follow that path.

Climb the ladder at the end of the path.

Now on this floor, follow the linear path forward.

The portal with Reshiram is at the end of the path next to a ladder.