Grinder spawns after you kill the Bokon Maru.
Fastest way to spawn named if you're high enough level is to just run the the room with Bokon Maru and drop aggro (whatever class ability you have to drop all aggro). The update spot is at the entrance to the room. It's possible to run past it without triggering it. Just kill the sentinels and walk back to the entrance and the quest should update. Then just clear a path and kill Bokon Maru.
He's in the room that is the most east on the map (one of the central rooms). It's the same room Bokon Revel the Reborn can spawn in.
Edited, Jul 4th 2023 3:27pm by LyrikMindlethe