Journey to the 7th Gym

After the flight from Mistralton City, you should be at Yamaji Town.. There's nothing much here as well.. If you head toward the right, you will see a cave.. There's also a hidden path to the left where leads you to the Haunted Mansion.. Going there will help you to obtain Lunar Wing, but you won't be able to use it till Post Game.. There's two trainers and you will most likely meet Gothita/Solosis, Golbat or Banette.. So it's your choice doing this quest now or later..

So now let's go inside the cave.. Here is Reverse Cave, you will be tagged with Bianca.. She will heal your Pokes after each battles.. So you might want to spend time to train and explore here..

For B2: If you head east all the way from the place you entered, you will eventually see the exit to Undella Town.. However she doesn't want the exit, she wants to find a secret room.. It's located at the south from the entrance you entered.. Walk straight down till you see an exit (this exit will lead you to outdoor which has an item) then turn right.. You will eventually see a nurse standing beside the stair..

For W2: Here, it's magma-ish cave.. The cave is also far more complicated than in B2.. But just like B2, the secret cave is located at the south area.. The exit will be in north-east area, which would means you have to walk one counter-clockwise direction to reach there.. However she doesn't want the exit, she wants to find a secret room.. It's located at the south from the entrance you entered..

In fact Bianca should hinted you something like "It's here!".. Go down the stair and let Bianca investigate.. There's nothing but an empty room.. This will be the place where you can find Heatran in the Post Game after you got Magma Stone.. So remember this place well.. Bianca will still follow you, so feel free to explore the whole cave.. After you are satisfied exploring, use the exit I mentioned earlier on..

Now, you should be at Undella City.. If it's Monday, you will be able to see DW Jellicient on the sea.. You can explore the south area, there will be a few trainers for you.. However you won't be able to enter Black City/White Forest yet.. Once you have beaten all the trainers there, go back to Undella Town and head upward.. Your rival will ambush you..

Go through Route 13 till you reach Lacunosa Town.. Before you reach the city, you should see Cobalion.. It won't run away, so you can go to PokeCenter in Lacunosa Town and come back to get it.. It will be at lvl45.. In Lacunosa Town, you will see your rival there.. You will then have to tag battle with your rival to defeat Zinzolin and his underling.. After you have beaten them, go through Route 12 then to Village Bridge.. The first house from the right on the bridge has a healer, so you don't have to fly/walk back to PokeCenter.. Next go through Route 11 till you reach Opelucid City.. Of course, take your time to explore all the 3 places.. On Route 11, you will see Virizon.. Just like Cobalion, it won't run away.. You can catch it anytime you want..

Village Bridge (Secret Singing Room)
Under the bridge, on the left side, there's a secret room where you can enter.. Here, you will meet a girl singing, who will then chase you out from the room.. Go to the top of the bridge, directly above the room, talk to the girl who is asking where this mysterious voice coming from.. You will then receive Citrus Berries from her as a reward..