To be honest, I could’ve done this a lot sooner. The 3 “red merits” (legendary merits) cost 30 Champion merits (silver certs) and 9 Supreme merits (gold certs) total, which isn’t terribly steep. The special Bahamut horns have an extremely high chance of dropping from successful Proto Bahamut Impossible (Bahamut HL) raids, so it shouldn’t take more than 5~7 good Bahamut HL raids to obtain them. In a way, upgrading a Bahamut weapon to Coda is easier than the SR -> SSR upgrade step.
I’ve been reluctant to run Bahamut HL for a long time because my light teams are weak and wouldn’t be able to contribute much. My dark teams have better firepower, but the last 50% of the raid is more dangerous for them. Rolling the Onyx Carbuncle in mid-October was crucial for survival, and what convinced me to play Bahamut HL more.
Anyway, the Coda Bahamut Axe comes with a 15% HP increase for Draph/Harvin races, which benefits 3/4 of my main dark team (the player character, Narmaya [AKA Naru], and Vania [Vampy]). My dark teams have struggled with survival in harder HL raids because of the low-ish HP, so this helps greatly.