Leaning Iron Mountain has stable damage due to getting through defense, but it's lackluster in cases where your ATK or MAG stat already significantly outclasses the enemy's def/mnd. This becomes more apparent in postgame where your attacker's atk/mag are most likely, slowly getting pumped farther and farther ahead of other stats due to equipments and levelup bonuses. Also because you're likely overleveling to smash through bosses since it's not too hard to reach lv180-ish, which is around the level to get to for the strengthened final boss, the last thing to fight.
I never actually tried the WND attack on Monk, though. It's single target, unlike how the Sorc and Warrior's less interesting skills are clearly designed for AoE random battle use, so I could understand it actually being pretty nice. Speaking of Warrior, I really should have used Explosive Flame Sword more, since I was using Mokou tank a lot (blazing passive) and many bosses were weak to it, yet I had -no- source of FIR on any of my damage dealers. But after a bit I phased all uses of warrior out of my party in exchange for other subclasses or for characters who wanted something else more...
Sorc subclass skills are basically only good for adding elemental variety to your skillset, that much I found out. Several magic users can get good use from that, though, and their single-target is still okayish damage for hitting bosses, so that's fine. Just nothing special.