Be very careful when coming to this hunting place, because there is a large number of Grim Reapers and other creatures. A single Reaper can combo over 1500 damage per turn, if you are a knight lower than level 150 it may be very dangerous. Mages and paladins should get a blocker before going there.
To get in there you must go to Drefia. Near the centre of the mountain wall, there is a little ruin that has a Zombie inside of it. You will need Levitate or Parcels to get there. Go down the stairs where you will face a few floors of minor creatures until you enter a room with wide sandstone stairs. Down these stairs are the Grim Reaper Dungeons.