Mission 15 stone is on a mesa in the middle of the central plain and it requires riding a Chocobo. From mission 9's Waystone, ride the Chocobo toward the Moon, you'll see a lake on your right side, pass this lake is the mesa we talked about. Hop on it then hop 2 more times to the mesa to the far right you'll see the stone.

This is like mission 13 but has more Goblins so the strategy is pretty much the same.

• MISSION 15: Goblin Chieftain (x1), Goblin (x6)
  Reward: Survivalist Catalog (x1)
  Strategy: 
  • Try your best to Preemptive Strike because you will want to kill those Goblins as quickly as you can but it's okay if cannot do it.
  • Start with COM/COM/RAV and use Ruinga/Ruin to kill Goblins first. Once Goblins are out, switch to COM/RAV/RAV to finish the Goblin Chieftain one by one.
  • Switch to SEN/MED/MED if you need healing.

 Mission 16 also requires a Chocobo to access the stone. Just open your map and pick one up near you. From where you fought mission 15, keep walking along the left walls until you see a Save Point then walk into the valley guarded by a tortoise. The stone is on the top right of the hill guarded by a cactus monster but you don't have fight it.

• MISSION 16: Ceratoraptor (x4), Sahagin (x2)
  Reward: Rhodochrosite (x1)
  Strategy: 
  • Again, Preemptive Strike is always your best bet.
  • Start with COM/COM/RAV and focus on Sahagin first. Switch back and forth between COM/RAV/RAV and COM/COM/RAV and kill Ceratoraptor off one by one.
  • Switch to SEN/MED/MED if you need healing.
  • Switch to SAB/MED/SYN for 1 or 2 turns is also good for sticking Slowga on all of them.

Pick up a Chocobo. Mission 17 stone is at the far north (Moon is north) of the Archylte Steppe. When reach the central plain, walk toward the Moon until you reach the north, you'll see a Behemoth, a Chcocobo, a Save Point, and a path going down hill leading to another area. The stone is on the path and you'll have to walk pass 2 giant monsters fighting each other. This Cie'th Stone is also a Waystone.

• MISSION 17: Ceratoraptor (x4), Sahagin (x2)
  Reward: Perfect Conductor (x3)
  Strategy: 
  • This one is very easy to Preemptive Strike.
  • Start with COM/RAV/RAV, it should go down in one stagger.

We are done with Archylte Steppe for now, it's time to move on to progress the storyline. Head back to the path where you activated mission 17 Cie'th Stone and proceed to the next area Mah'habara where you first grinded your Crystarium Points.

Reminder to spend all your Crystarium Points on your primary roles.

Keep moving forward and skip the left path for now until you reach a lone Pulsework Centurion and 2 side areas. Inside are treasures.

• ITEM (T-01): Electrodes (x4)
• ITEM (T-02): Chobham Armor (x4)

Follow the main path a little further, a cutscene will occur. when you walk a few more steps, you'll see a yellow Cie'th Stone on the mini-map which is the mission 18. Good news is the Mark is close by. This is also a Waystone.

• MISSION 18: Ambling Bellows (x1), Hoplite (x4)
  Reward: Sorcerer's Mark (x1)
  Strategy: 
  • As you can see they are all waiting for you, so Preemptive is impossible for this one.
  • Start with SAB/MED/SYN and focus on Ambling Bellows first, Once the debuffs stick on it, switch to COM/RAV/RAV and finish it off and Hoplites are next. DO NOT kill Hoplites first as Ambling Bellows will summon them indefinitely.
  • Heal if needed.

Once you defeated the Ambling Bellows, keep going forward.

• ACCESSORY (T-03): Saint's Amulet (x2)

There is a Save Point ahead before you fight another Boss, well it's the last Eidolon.

• EIDOLON (B-01): Hecatoncheir (x1)
  Strategy: 
  • Use SAB/COM when you see "Looming Wrath" on his head. Once Deprotect, Deshell, and Imperil stick, switch to RAV/COM.
  • If he uses something else other than "Looming Wrath", switch to MED/SEN for defense because he is gonna hit you pretty hard.
  • Always keep your health bars green.
  • Repeat until the gauge is full. 

Continue the main path until you see another treasure.

• ACCESSORY (T-04): Ice Charm (x1)

• ITEM (T-05): Particle Accelerator (x1)
• ITEM (T-06): Crystal Oscillator (x3)
• ITEM (T-07): Perfect Conductor (x3)

Once you reach the yellow mark, hop on the giant wheel (Atomos). When you arrive your destination, you'll see a Cie'th Stone, that is the mission 19 and it is also a Waystone. Before you go to the Mark, there is one thing you need to do. There are 4 hidden treasures that require you to trigger a cutscene to raise 2 islands. Once you reach second platform, you'll notice there is a right path leading to the Mark, but instead go to the left side and hop on a small area you see on the mini-map. The "EXAMINE" button will appear.

Once you raise the islands, go back to previous platforms and collecting items along the way to mission 19's Mark.

• ITEM (T-08): Uraninite (x1)
• ACCESSORY (T-09): Riptide Ring (x1)
• ITEM (T-10): Strange Fluid (x13)
• ITEM (T-11): Enigmatic Fluid (x10)
• ITEM (T-12): Mnar Stone (x1)
• ACCESSORY (T-13): Water Charm (x1)
• ACCESSORY (T-14): Aquabane Brooch (x1)
• ITEM (T-15): Moistened Scale (x6)
   
• MISSION 19: Uridimmu (x4)
  Reward: Cobaltite (x1)
  Strategy: 
  • I don't think you can Preemptive Strike this one.
  • Start with COM/COM/RAV and Blitz them to death.

This one guarded by the mark:

• ITEM (T-16): Seapetal Scale (x5)

Follow the yellow mark and pick up 2 more items along the way:

• ITEM (T-17): Abyssal Scale (x7)
• ITEM (T-18): Cie'th Tear (x1)

The path is linear and the next mission's Cie'th Stone is also on the way. Note that Mission 20 is another Waystone. The Mark is just further down the path.

• MISSION 20: Goblin Chieftain (x1), Goblin (x2), Munchkin Maestro (x1), Munchkin (x1)
  Reward: Rhodochrosite (x1)
  Strategy: 
  • Preemptive Strike is possible but it's hard, still I recommend you try to Preemptive Strike.
  • Start with COM/COM/RAV and then throws Ruinga to stagger them all. Then kill smaller and weaker enemies first from Goblin -> Munchkin -> Munchkin Maestro -> Goblin Chieftain. I think you are strong enough to kill them all without the need of healing.

Then pick up an item in front of you:

• ITEM (T-19): Sinister Fang (x20)

Our next journey is Taejin's Tower.