I haven't gotten around to training any of my characters extensively as Gladiators (Paladins already do a far better job of filling the tank role) or Armamentalists (though I expect that'll change very quickly once I get to a point where I need a proper support character; for now, Paladins have both Kabuff and Magic Barrier for defensive buffs).
Also, regarding metal slime hunting: the (über) falcon blade in conjunction with Metal Slash is wonderful, since more often than not the skill will do four damage thanks to the double hits from the sword, and more hits also means more chances at one of them being a crit (I'd say Falcon Slash instead for the potential four hits, but it's not as good against metal slimes as the move designed for fighting them).
Have you unlocked the Luminary vocation yet? I have yet to find the person who gives the quest for it, and it's the only one I haven't unlocked.