Enter the Tomb of the Old Ones
Rokas is a good family member to use for the first part. Note that there are rooms where you will have to clear all enemies to get keys for unlocking doors. Although you can "cheat" by buying keys at the base camp shop if you really can't find them, it is more fun to locate the keys within the Tomb.
The Temple in the Sky
Rokas steps through the Portal to of all places...the sky! This first area is fairly straightforward, and you should get through it with little trouble. Still, if you haven't purchased a buckler for Rokas at the base camp shop yet, you should do so before tackling this temple.
You will be teleported back to base camp after you beat the first boss. With the new Wind Gear in hand, head to the room across from the Portal and cross the blue chain bridge there to enter the second dungeon.