Map: Valley of Seers
Recommended Element: Lightning
Heart Container: Lana
Heart Piece: Sheik
Skulltula: Defeat 1,000 enemies
For your main objective, you have to capture the two vital keeps to the west and east. Meanwhile, Impa will encounter Volga and a darknut on her way to the eastern keep. You can head there and defeat the two of them instead for potential material/weapon drops. They'll reveal their weak points right after their attack so make sure to evade and close the distance immediately so you can take the opportunity to smash their weak point and deal extra damage.
Once the two are defeated, move out and head towards the keeps. These Keeps have Beamos emplacements, which are like laser turrets. Destroy them using bombs. Capturing the West Keep will reveal a chest containing a Sealed Weapon.
Continue killing enemies for the meantime until the objective is updated. You'll have to escort the allied engineer on his way to cut down the dark vines protecting the ruins entrance. Head towards the engineer's path and take out the marked enemies that will intercept him.
After getting rid of the enemies, the engineer will be able to lower the East Ruins Bridge. You can head there or spend some time reinforcing your allied strongholds getting swarmed by enemies and darknuts. Head towards the East Ruins Bridge and shortly after, a small group of Summoners will appear. Backtrack and fend them off to help maintain your allied position in the area.
You can approach the ruins from either the west or east sides since they both lead to one of the keeps inside the ruins. Defeat all enemies inside as well as the Keep Boss to reveal a chest. This contains a new tool, the Boomerang. You need to use this to cut down the dark vines blocking the way. You can also head to the middle of the ruins and defeat the Gatekeeper. This will open the gate in the middle of the ruins.
Once you're ready, head to the Summit and confront Cia. After the scene, the scenario boss will appear. While you're in the summit, you can capture it to get more Rupees. Head to the middle of the map and fight Manhandla.
Equip your Boomerang and fight the monster. You need to hit it with your boomerang when the heads turn green. This usually happens only when the boss attacks. It's easier to hit the four heads at once when the boss prepares for its bite attack. Once knocked the heads are all knocked out, it will expose its core. Attack the core until you completely deplete its Weak Point Gauge. After that, the Manhandle will burrow and sprout 5 stalks. Head out and defeat all of them using your boomerang until the main body appears. Repeat the same process until you completely defeat it.
Once defeated, the party will split into three scanario paths. Impa and Sheik will take the top route, Lana will take the middle, and finally Link will take the bottom path. These stages are different eras and dimensions of previous Zelda games.