Well, if you still can't do it, here's the most comprehenisve guide I caould arrange to help you getting one.
If you do either Endless Nightmare #2 or Mop-Up Operation #2 on ULTIMATE you may earn one.
Endless Nightmare #2 gives 2 Bone Badges and Mop-Up Operation #2 gives 4 Bone Badges.
Once you have 7 Bone Badges, go into Towards The Future QUEST and talk to the WEAPONS member standing near the HOSPITAL. In trade for 7 Bone Badges he hands you a Lv.0 PANZER'S TAIL.
If you're a HUNTER and would like to raise mainly POW and DEF, SOL ATOMIZERs are great for the PANZER'S TAIL.
I sure hope that helps, and happy hunting!