Quest

A Quest is a mission given to the player by Mission and/or Boss NPCs, rewarding stat points, and Zeni, which are needed to progress through the game. In order to level up faster, once the quest guide beam, which marks the player's next boss, points to the one you are fighting, go to the next.

In order to accept the quest/mission and fight the Boss NPC, one must meet the set stat requirement, which does not seem to include defense. For the locations of quest givers, visit the NPC page. You will find at the bottom of that page a map of the locations of each NPC.