Let's go back to where you activate mission 5's Cie'th Stone. I know it is a lot of back and forth but the Sixth Cie'th Stone is on that way. When you arrived the fifth Cie'th Stone, open your map you'll notice another path on the left not very far from the Save Point, so take the left path until you reach a cutscene and the Stone is right in front of you. By the way, this is also a Waystone.

• MISSION 06: Munchkin (x4), Munchkin Maestro (x1)
  Reward: Fulmen Ring (x1)
  Strategy: 
  • I don't think you can Preemptive Strike this one, so let me know if can do it.
  • Start with COM/COM/RAV, then use only Blitz until those minions are gone, then use Auto Battle to finish the boss. You don't even need to switch paradigm in this battle.

Don't forget to pick up the treasure.

• ACCESSORY (T-01): Mythril Bangle (x1)

Head back to the 6th Cie'th Stone and go in to the archaeopolis, take the stairs until you see 2 rows of columns, that's where the stone located. There are 3 treasures in this area.

• ITEM (T-02): Rhodochrosite (x1)
• ITEM (T-03): Spined Horn (x3)
• ACCESSORY (T-04): Whistlewind Scarf (x1)
   
• MISSION 07: Bituitus (x1)
  Reward: R&D Depot (x5)
  Strategy:
  • Unfortunately, you cannot Preemptive Strike this guy though. 
  • Start with MED/SAB/SYN. Once Fang stick Deshell and Slow on it, switch to RAV/COM/RAV.
  • If you are low on health, switch to MED/SAB/MED
  • If it cast debuffs on your party, switch to MED/SAB/SYN to clear the debuffs.
  • Repeat until it dies.

You are about to be done on this part of the map for now, let's go talk to mission 6 Cie'th Stone and teleport to "Yaschas Massif - Tsubaddran Highlands", then walk toward "Vallis Media" until you see the 8th Cie'th Stone at a 3-ways intersection. This is also a Waystone.

• MISSION 08: Rakshasa (x1), Flan (x3)
  Reward: Collector Catalog (x1)
  Strategy: 
  • Lure them out and when they walk back, rush behind them to trigger a Preemptive Strike.
  • Start with COM/COM/RAV and Blitz them until the 3 Flans are dead then switch to RAV/COM/RAV. Once it is staggered, switch back to COM/COM/RAV.

There is an optional cutscene here. If you notice a yellow mark on the map, it points to Yaschas Massif area. The yellow mark is actually close to the mission 5 Cie'th Stone. Just follow it until the cutscene is triggered.

Once you watched the cutscene, use any closest Waystone and teleport to "Vallis Media - Base Camp". From there, head back to Archylte Steppe for our next mission.

• MISSION 09: Kaiser Behemoth (x1)
  Reward: Rhodochrosite (x1)
  Strategy:  
  • Lure him and walk around to sneak up for a Preemptive Strike. Without Preemptive Strike, it gonna be super tough to get 5 stars.
  • Start with COM/RAV/RAV to finish it off.