There are 4 personas that gain skills no other persona can: Surt(Ragnarok - fire), Odin(Thunder Reign - Lightning), Norn(Panta Rhei - Wind), Skadi(Niflheim - ice). These skills are the Severe version of each of the elements, and can do massive damage if put on a persona that has boost + amp of the appropriate element, especially boosted.
You can fight the Reaper the regular way, but the "cheap" way(so long as you're playing Fes) is to fuse an Odin and level it til it gets Thunder Reign. Thunder Reign has a 100% chance of shocking an enemy, which means a physical attack will knock it down. MC uses it, then the next character uses a physical to knock it down. Refuse the all out attack, then continue. Reaper wastes a turn standing up, and you can beat it this way without ever being attacked.
You CAN beat it at your level normally though if you keep your party buffed and Reaper debuffed, but it would take awhile. Only real award for doing it is opening up a new block in Tartarus that has insanely high leveled mobs(but because of this your party will gain 2-3 levels a fight you survive at first), so you can get even more ridiculously overleveled.