Umm, I think I was around level 40. My moves were doing 50 damage apiece. I thought I was doing pretty good until he dropped a 1300 damage deuce on my party. I had to redo like an hour of grinding. --- Check my FictionPress: http://www.fictionpress.com/u/882728/ I love Rias Gremory.
^ May I ask what's your level? I was level 83 with my party 80 on Normal. I only fought the Reaper twice but it seems luck based on his moves. I remember, all it did was cast break spells on my characters and Magarudyne. Second time, it did back-to-back Megidolaons, then Maragidyne, Yuki gets downed and it nukes us with Megidolaon.
Just pray the RNG gods decide to not have the Reaper use "All-In" tactics. ---
My MC was level 80. Everyone else with me 75-76. Im leveling up quick so by the time i see him again ill have better Personas and moves and maybe find the level cap.