I still do not think you could have killed him before the Phon Coast sidequest. But, if he's dead, I highly doubt he will respawn. Sort of like a missed trophy chance, like killing Omega and not grabbing the badge, it's gone forever. Considering that, I would not recommend farming Souls of Thamasa from Oversouls, I would recommend killing rare game like Greeden on the Dalmasca Estersand. High Arcana's and Souls of Thamasa both have a 5% monograph/Canopic Jar drop rate, so it seems more wise to try and farm High Arcanas. I farmed the Oversouls and it took me 3 hours, that's not the way you wanna go. I hope this helps you.