This hunt becomes available any time after you have completed the Ancient City of Giruvegan and the Great Crystal. You should return to Balfonheim Port to speak to Reddas and have him join your party before starting this hunt as having Reddas in your party will be helpful.
Miclio is hiding in a doorway in the southernmost section of Miner’s End in Bhujerba. Use the map and the screenshot below to help you locate him:
Note that you will require the Site 11 Key for the Lhusu Mines which can be obtained at the Hunter’s Camp in the Phon Coast after completing Hunt 38: Antlion Infestation (Antlion - Rank V).
At the end of Hunt 38, the kids will drop a key down below which then appears in the Hunter’s Camp next to the Fallen Bhujerban NPC. Use the screenshot and map below to help you find it:
Warp into the Lhusu Mines orange Save Crystal in Transitway 2. Head west through the Tasche Span and into Site 9. Head south and follow the pathway, turning right when you can and approaching the large gate.
Use the Site 11 Key on the gate and proceed through. Diabolos is located just to the west of where you enter Site 11 from Site 9.
Level: 46
HP: 93,551
Steal: 
Fire Crystal (55%), Demon’s Sigh (10%), Scorpio Gem (3%)
Do not let Diabolos’ low level and low HP throw you off. He is extremely difficult and can easily turn the tides of the battle in his favor no matter how well you think you are doing.
Once Diabolos’ HP falls below 60% it will begin building up an auto-counter barrier and countering all of your attacks. As such, the key to this battle is speed – bring Diabolos down quickly as he becomes more and more difficult to defeat the longer he is alive.
One of Diabolos’ moves is called Curse. It can inflict the Poison, Disease, Sap and Confuse status effects on all party members. Equip your party members with a 
Bowline Sash to prevent the Confuse status effect and set up Remedy Gambits (rather than 
Esuna Gambits, 
Remedy items are faster to use) to deal with the other status effects.
Make sure that your party is fully buffed before you begin the fight. 
Bubble, specifically, will prevent the Disease status effect from taking hold. Shell is also critically important due to the amount of Magick damage Diabolos is capable of doing.
Set up a Gambit to keep 
Slow on him the entire fight as he is vulnerable to that status effect. You should also have one of your characters Dispel the Haste status effect that he casts on himself. Spread your party members out to ensure that as few of them as possible are hit by Pyromania, an extremely powerful Fire-based Magick spell.
A video demonstrating the fight is included below:
Head back to Bhujerba and speak to Miclio when you are finished. He and the other Children will thank you and will give you 2,600 gil, a 
Demon Shield and a 
Zeus Mace as a reward.
This Hunt is normally completed after Reddas has joined the party when they return to Balfonheim Port for the second time. Click on the link to return to that section of the walkthrough.
