The following are required to boost your experience and DEV: aStatus 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 receive tons of EXP and DEV, especially to the ones with level 50 higher than others. That character can earn 20+ EXP and 15+ DEV by casting a Status Relief Coin or killing a single enemy with one hit.