From Granblue Fantasy Wiki
|Purchase a Class Champion Replica Weapon||1|
|Upgrade Bahamut Weapons to Coda form||3|
|Upgrade Seraphic Weapons to SSR form||2|
|Crafting Golden Relics for Eternals 5★ uncap||5|
|Create an Ultima Weapon||10|
|Uncap Premium Gala Weapons to 4★||2|
The easiest way to obtain Legendary Merits is to exchange 3 Supreme Merits and 10 Champion Merits for them in the shop once a day. The only other way to obtain it is as a rare drop from Proto Bahamut HL or Ultimate Bahamut HL.