The deepling brawler can be found in Fiehonja in the area where you can complete the Soul Net mission in the Liquid Black Quest. There are not many in the spawn, so I would recommend killing them on rapid respawn for an hour and rate how many you kill/hr. If you feel as though you kill enough in that time frame a bestiary potion could assist in finishing the bestiary. Expect to be in the spawn for a while. Happy Hunting!!