Efficiently gaining JP and experience is crucial for character development. The 'Gained JP Up' support ability (Squire Job Level 2) is a must-have for all units. Equip it as early as possible. Random encounters are excellent for grinding. Look for maps with easily defeatable enemies or maps where you can isolate one enemy and repeatedly use 'Accumulate' or 'Chakra' to gain JP without ending the battle.
Another strategy is to use the 'Stone' status effect (from Geomancer's 'Stone' ability or Medusa's 'Gaze') on an enemy. A petrified enemy won't act but still counts as an active unit, allowing you to safely grind JP by repeatedly using abilities on your own units (e.g., healing spells, 'Accumulate'). Be careful not to accidentally kill the petrified enemy.