When you enter Stelumbra Cave, you learn about Curios. The in-game tutorial can be a little confusing, so...
Basically, there's a Curio at the entrance to each dungeon, in the middle of most dungeons, and at the end of quite a few of them. When you touch the Curio at the entrance to a dungeon, you can set the encounter rate to Half, Normal, or Double. In the middle of the dungeon and at the end, you have the added option to set the encounter rate to None or Triple.
You can also Warp to the entrance of any dungeon that has a Curio, as long as the "Warp" function is not X'ed out.
Also, when you touch any Curio, you can "call" 3 consecutive battles immediately - or you can spend 10 Gems to summon one group of jars, or one group of blocks. (I've never used any of these features, because later on there are other options for encountering more jars or blocks.)
You can also buy a "Portable Curio" in the IAP Shop when the Shop becomes available, but in all honesty it's not really worth it. (All it does is allow you to set the encounter rate to None at the entrance to a dungeon.)
A few steps into the cave, you run into Toril and she joins your party again.
She also gives the party two "buddies," and your party is now separated into "Teams" in every battle.
Essentially, instead of 3 party members you now have 6 - but those 6 are split up into teams of two.
Each team has a main member, and a sub.
The main member can attack, use skills or magic, or defend (note: if the main member defends, the sub can still use items or skills, but cannot use a normal attack.)
The sub can "support" the main member, making their attack, skill, or magic stronger; they can use skills or magic of their own; or they can use support items (healing potions, resurrect potions, etc.) Buddies can only be in the sub position.
The main member's action is chosen before the sub; however in battle, the sub will act first. It seems confusing at first but you'll get the hang of it pretty quickly.
To set your teams, go to Menu->Tactics->Formation. Now tap on the party member you want to move, then tap on the space you want to move them to (main is on the blue square, and sub is behind them). You can also move the whole team from the Vanguard to the Rearguard by tapping on the main, then tapping on the blue square in the Rearguard.
For more details on Team set-up, see Easy Team Set-Up, or check out the Fernz Gate Forum.
On with the game, already!
Now that we're done with all that...
Collect all the treasure on the first screen, and make sure you talk to the Archer Woman in the southwest corner.
She'll explain buddies and buddy rings, and then she'll give you a Cool Chocolate just for listening to her!
On the second screen, there's a secret passage right near the entrance.
This one is on the "bottom" of the path, instead of on the right or left, so it's a little bit harder to spot.
But once you know what the "bottom" secret passages look like, it's easier to spot them in the future.
Unlocking the IAP Shop
Along the southern path on the second screen, you run into a group of adventurers who tell you about Jar Breakers in the IAP Shop. They also give you 1 Premium Ticket to use on equipment, and 1 Premium Ticket to use on weapons. (Basically, it's a chance to win a really cool free weapon, or piece of equipment, or a premium Buddy.)
Note that the prizes you get are random, so if you're not happy with the results, you can reload your game and try again.
Also, the IAP Shop unlocks, and you can now buy premium items or play Jar Breakers any time you want.
Most of the items in the IAP Shop are not necessary in order to complete the game or all the bonus material - however, there are 8 storehouses scattered throughout the towns and villages, and you'll need to buy 6 Storage Keys if you want to be able to open them all.
You'll also need to buy the Laboratory Key in order to enter Logus Laboratory (which is an optional, bonus dungeon, but is loads of fun).
If you're not sure where to spend your Gems, you can also check out "Where to Spend IAP" in the forum.
Now Back to the Story
Collect the remaining treasure on this screen. At the northern edge there's another Curio, and touching this one will let you set the encounter rate to None, if you so desire.
East of the Curio is another boss battle.
Sword Master x2
This is another really easy battle.
Just be sure to use Toril's Brave Strike so you can steal all their drop items.
After that, use Kodan and Lita's Brave Strikes to finish them off.
(Although, you could probably put this battle on Auto and still win. It just wouldn't be as satisfying.)
Once the battle is over, watch the short scene and then exit Stelumbra Cave and enter Rizolt City.