Various prequests are required to unlock hard magnus as a weekly boss.
You can access hard magnus after completing all quests via the party window then boss tab and magnus icon. Hard magnus can be completed once a week where as normal and easy can be done daily.
To start the quest go to your white lightbulb on the left side of your screen and accept the (Lv.90) To Pantheon – Beldar prequest
After that quest follow along the following quests below to complete the hard Magnus prequests.
(Lv.90) Supporting Heliseum Reclamation HQ – Beldar
(Lv.90) Heliseum Reclamation HQ In Danger – Tiron
(Lv.90) Edea and Piston – Edea
Eliminating Battle Hounds – Piston
(Lv.90) Securing the frontlines – Edea
(Lv.90) Sturdy Foundation – Edea
(Lv.90) Finding Emergency Supplies – Harpoon Quartermaster
(Lv.90) Dispatch Mission – Edea
(Lv.90) Anger Management – Eurenth
(For Eurenth’s advice you have to head to Forbidden Forest Secret Merchant Meet-up map and go to your use tab in your inventory and click on the D-03 Transmitter to activate the quest)
(Lv.90) Grandis Shadow – Tonero
(Lv.90) Freeloading Trade King – Romero
(Lv.90) Tonero’s Request – Tonero
Information Delivery – Eurenth
Go back to Heliseum Reclamation HQ if you havent been teleported there and speak to Edea in the command barracks.
(Lv.90) Seizing The Ores – Edea
(Lv.90) Operation, Go! – Edea
Castle Crash – Edea
Complete the quest via lightbulb in your head and a new quest will appear once again above your head
(Lv.102) Nova Troubles – Edea (Final quest)
Congrats you have now unlocked Hard Magnus!