If it's the boss I'm thinking of, you'll want to take off any counter attack items you have on. Switch to your strongest characters (Random, Sarah, Luciana, Coryool, if necessary). I can't remember which elements this boss was immune to. In any case, first round have Random cast Wrath, Sarah defense up all, Luciana protection, have Coryool just bash it up.
Next round Random attacks the Spirit of the Manor, Sarah casts attack up (should go in effect before Random attacks as her speed is higher

), Luciana casts a spell which attacks all to get rid of the little guys, (fire dmg on all) Coryool continues bashing it up. After that, just attack normally and use a high dmging spell with Luciana. Heal where necessary. I had no real problems with this boss, except when I forgot to remove counter attack items. Cuz you keep hitting the boss back, but he counter attacks too, but his HP is too high to have a counter attack battle (unless you have baretta... lol... but Baretta can't be with you unfortunately, later on Baretta is even tougher to kill than Random with the right items, I remember a yellow colored slime boss in the Cosmopolitan Hotel that killed Random in a counter attack battle, but Baretta killed him even though her attack power was way lower, cuz she only took like 1 dmg per hit from the boss (9 if it was a crit ) but got healed for 26 points each round rofl) Grandiose Axe + Formelhaut Armor = Baretta's deification rofl