This maybe late but here is the trick to fast farming merc exp (account exp) without grinding tiny daily exp day by day or rerun Shadow event
Highly recommend farming with x4 reward or more (x8~x10 for most effectively). Also use emulator if u dont wanna melting ur phone with ridiculous "BBQ" temperature from the game running
Credit to the guy who wrote this in English although this trick is already being exposed very long time ago on other servers