With this particular job, it seems that there’s an odd problem with one of the rare and valuable Gauntlets. You need to head to the treasure room first - first to Mikado Castle, then to the Treasure Room. Confirm that you are prepared, and more info will be divulged by the master samurai.
A powerful demon resides in it, interfering with its operation. The Gauntlet must be cleansed of such demons before an inexperienced novice - such as those in the Rite - peruse it. Of course, the demon within does not particularly want to leave its crib. None of the samurai so far have managed to forcibly remove him, either.
The Gauntlet is activated and a gladiator-like demon rises from within - Ares, named for the god of war.
While this God of War may only have about two-thirds of the HP of our previous boss, he also has much higher Strength and no true weaknesses, making him a formidable foe. He also has the attack Fatal Sword, which almost certainly will kill a party member if it strikes due to its high damage.
He also the first foe to be able to counterattack. This fight is best done in an all-out assault, especially when below Level 10. At pretty much any reasonable level, the threat of Fatal Sword will loom painfully over your head, making all attempts at healing or reviving full of vain.
If you happen to have the Media Skill, that’s the best that can be said for healing, but that would be about it. Just grind his HP down. Ares can be statused (except Lost), albeit a little tough if not Panic, so while you may want have that as a side-effect, by no means should be it become a full strategem.
After the battle, Ares willingly leaves.