Check if your equipment is up to par with the level of the job quest und use buff food (can use the cheap one you can buy from NPC'S). Each job quest can be done directly at the level you are getting it. Beeing a few levels higher makes those quests a tad easier but most of the time it is a matter of tactics like for example who you attack first or which NPC needs to be healed with priority. I only had problems with a few jobquests on the four classes I played so far and each single problem was solved after either talking to people from my FC or after watching a video.