Generally the best way to obtain the Assault Jerkin is to have a Thf somewhere in the range of 60-70 for Treasure Hunter, only one is needed. Myself and my friend who was 72Dnc/36Thf at the time and myself as 68Thf/34Bst for the Widescan. We were able to easily duo Ose without many issues aside from the near constant Paralyze effect. The best way to claim him is to hang around the narrow tunnel, which is the entrance to the larger cavern where he spawns and pick off the mobs until he pops.
And truthfully after just going 1/5, I'm not very likely to go back to Onzozo for quite a while.