The Mercurius first appears before the party as a boss in the Well of Souls in Mysterious Unison. It is later found as a roaming monster within the dungeon in groups of two or three.
Its normal physical attacks are foot stomps and several pecks with its beak that carries a chance of knockdown. Incinerate is two hits of fire on a target and anyone standing right by it, though if Guarded against, it will only hit once. It will telegraph that it is about to use this attack by grooming itself. Tail Swipe is a tail strike on anyone standing behind it. Finally, Suffocating Blow is a six-hit attack on all targets at melee range with a chance of knockdown and poison status. It only uses this attack at the beginning of a turn and telegraphs that it is about to use it by raising its head and flapping its wings prior to its next turn. If, however, all party members are more than a few feet away from it, then the attack will simply miss. If it doesn't seem likely that the party will defeat it before its next turn, then move everyone away.
Mercurius has some fairly tough moves, but like any other monster of its type, defeating it is all about knowing how to read its attacks. The Suffocating Blow is by far the most dangerous move, but as long as everyone is moved away from it prior to using it, then it won't be able to hit anyone. Its motions also make it fairly easy to predict when it will use the dangerous Incinerate.
As a boss, it has reasonably high HP, but using strong characters and attacking it with Special Attacks and Harmony Chains will finish it off fairly quickly. Claves can be put in the party for the first opportunity to level her up since she left the party, but this boss really doesn't give much EXP. If she is placed in the party, she should be moved to the edge of the battlefield and simply use Unicorn Horn a couple of times every turn.
Fighting them as normal opponents is fairly easy since the player will now have Party Level 6 (if they didn't have it already in an Encore Mode playthrough.) Simply unleash a long series of Harmony Chains and if they group together, Echoes can be built up quickly. With a powerful party, they may easily fall within a single round.
- In the PlayStation 3 version of Eternal Sonata, this monster's HP was increased for its boss form, but in the Xbox 360 version, there was no increase. Additionally, in the PlayStation 3 version of Eternal Sonata, the amount of EXP granted for defeating this monster was reduced by over 75%.
- In a normal playthrough, Party Level 6 is granted for defeating the boss form of this opponent.