To be honest, Absorb MP and Faith Up triggering off anything that uses MP allows you to chain it to stuff like battle song. Since this lets me build better synergies, I see no reason to change that.
Stone Gun isn't really broken right now. The damage is only average, the range is OK, but on a small map, your team loses the tempo of 2 attackers.
Madlemgen's proc should be changed to Magic Ruin so it would be more useful (while having the same overall effect)
Change Ultimus bow to 15 WP, +1 PA. This would at least guarantee some level of use.
Change Windslash bow to 12 WP, Wind Elemental, 19% Hurricane (-33% of maxHP, wind elemental)
Change Lightning bow to 13 WP, Lightning Elemental, 25% Don't Act
Change Ice bow to 13 WP, Ice Elemental, 25% Stop
Change Silver bow to 12 WP, Holy Elemental, 19% Holy
Possibly make longbow a 12 range weapon that adds don't move. Alternatively, it being 8 range would work too.
Consolidate Assassin's Dagger with throwing knife. Right now, neither is a good weapon, but combined, they might pose a real benefit to mediators who don't want to learn death sentence. Instead of Assassin's Dagger, add the Rusty Shank (8 WP, 2Sable, 50% +Poison)
Make Diamond and Platina shield 15/15 on evasion.
Give N-Kai null undead instead of confuse.
Give Jade Armlet null ice instead of null chicken.
Give Reflect Ring null berserk instead of silence. (It's currently overshadowing the magic ring a bit too much right now.)
Give Defense Armlet absorb fire.
Give Defense Ring some evasion (8/8 should do, I think).
Oil should add weak to more (possibly all) elements. Of course, this means it will be dispelled by attacks of all elements.
Secret Fist should be MA + 75% (subject to P-EV) to have any reasonable level of parity with the mediator skill.
Make Wiznaibus add poison (at 50%) so it's not entirely blocked by auto-potion. Make Nameless Dance add oil (see above change) instead of poison.
Similarly, have life song add regen (at 50%) so it's not entirely overshadowed by mimed murasame.
Make nameless song 4 CT and have it add reraise, protect, or shell at 33% chance. This would make it much likelier that it would be used to save a unit's life without causing the AI to never do anything else.