I finally got him and isn't Wrexsoul weak to fire/holy? I scanned him and got that info. Anyway, what worked was using Rasp since it triggered his special condition weakness and and will reset his counter for Possession so I managed to keep him stunlocked by having Relm using Sketch on him repeatedly to Rasp while Terra / Gau stayed on healing duties. Reducing his MP to 0 killed him, took around 10 Rasps to do it. Didn't know you could ignore his gimmick in this mod but hey, at least I got him.