How can I max black mage efficiently

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The black mage job requires alot of JPs.
I tried maxing it but sending only Ramza, I thought i would kill all enemies but one and send him retreating with low HPs so i could use focus.
However I am already level 50 so usually in a random encounter Ramza will have to face 5 enemies with 300HPs.
Is there a way or specific battle which could be used for this ?
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FFXIV - Black Mage Endwalker Guide - Openers, Rotation, Optimization, and Frequently Asked Questions
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