My party is all level 41 now, so after pwning the Bomb King, I made a suicide run into the Necro-place and finally, after days of fear, got the Zodiac Spear. To be honest, I am just happy to know I can open chests now without screwing myself.

As for the Z-Spear, it can almost one hit enemies that are level 38, so I would say it is worth a run once you hit 40 to pick it up. Getting through the Necro is a pain, but worth it.