You must have the following to boost your experience and DEV. Have a Status Relief Coin with the highest MCP, lowest CP and RC with an AOE of about 13 (if desired). Also have a charter with a skill of LUCK +10, 20, 30 or 50% skill and equip it on your character. Finally, cast it on your characters that can affect all of you. Even an enemy can help. You will then get a lot of EXP and DEV, especially to the ones with level 50 higher than others. That character can gain 20+ EXP and 15+ DEV by casting a Status Relief Coin or killing a single enemy with one hit.