Basically you need at least 3 stars of area affinity for the Satoryl Marsh/Forest/Nopon Village area before the necessary sidequest chain even becomes available. Then, you have to do a series of 3 or 4 sidequests in this chain before you finally get Monado Eater and the Advanced Manual for Battle Soul (allows you to upgrade it to Lv.10). If you feel like going for it, then you can start now.