About that EVERY DQ game that had job classes did that so that applies to the jobs in DQ 6 7 and 9, and the Japan only MMO DQ10 as well. But in DQ6 and 7 the weapons/armors were PARTY MEMBER NOT JOB class specific.
That depends on the lv if Demon Slash would take like 35 to 40 levels to learn it your still better off learning the Dragon Transformation spell Mages/Sages at lv 35 to kill more then 1 metal slime at once instead.