Run through Mosphoran and into the Salikawood. Head straight for
the Gate Crystal at Quietened Trace. Save your game. The Bomb King is one
screen away from you. Put on your game face and head north. The Bomb King will
start out with some minions, so get rid of those first. However, once you've
done so, pull back out to the crystal. The little ones won't be there when you
go back.

Kill the Bomb King. This is easier said than done, but you should be able to do
it with the help of Kotetsus, Demonsbanes, and your Francisca. Equip as many
of those as you can. As far as your characters go, make sure that you have at
least four characters that know MQs. This shouldn't be an issue by now, but it's
the bare minimum if you want to be safe, and for those four, it is preferred
that at least three of them have attained their Level 2 MQs.

Once your characters are set up, rush in. Hack away at the Bomb King until it
starts to call for help (reviving the little Bombs). This usually happens with
50% to 60% of its health remaining. When you see this, immediately start your
MQs. Sometimes your chain won't kill it immediately. If this happens, you must
not allow the Bomb King to stick around for much longer, since it will cast
Renew to heal itself completely, so swap out one character for your fourth
character with MQs and start another chain, hopefully one that will do enough
damage to finish it off. Obviously, if you have more characters with MQs, you
can repeat this process up to two more times.

Retreat to the save point again and rotate all of your characters around so
that every single one of them has an active Libra/Protect/Shell. Save your game.
Now, reform your party so that it has only one member.

Run through where the Bomb King previously stood and take the left branch. Keep
heading west and you will enter the Necrohol of Nabudis. This area is filled
with deadly traps and difficult enemies. With your level of equipment, it is
stupid to pick fights in here. That's not why we came, anyway.
Avoid all traps and run from all the enemies. In the first area, keep heading
west while searching for the exit. This is the hardest area to get through,
since Baknamys are extremely aggressive. When you lose a character (and you
will), swap out another one, but ONLY one. This guarantees maximal
survivability. Once you reach the second area, stop to revive, heal, and
recast Libra/Protect/Shell on everyone. Keep running, this time heading
northeast. Eventually you will reach Cloister of the Highborn, the central
area and home to what is for the most part the strongest weapon in the game.

Make a right from where you come in and then the next left. You should be
heading down a corridor heading north. Take the first left; this room should
have sixteen treasure chests in it if you left the right chests alone earlier
in the game. The Zodiac Spear is in the second chest from the left in the row
closest to you as you enter the room. You will likely have Elvorets on your
tail, so grab the weapon and get out of there. Backtrack through the first
two areas and exit out to the Salikawood. Make your way all the back to the
Gate Crystal. Congratulations!

Note :
if u want this weapon avoid the following chest (don't open it)

1. At the southern end of Rabanastre Lowtown, do not open the chest that
  sits in front of Old Dalan's house.
2. In the Rabanastre Palace Cellar, do not open the treasure chest(s) that
  sit in the southeast corner of the map. There are either one or two
  chests that may appear there. Do not open anything in either case. To be
  specific, do not open the chest with the Elixir in it (on your left as
  you approach the southeast corner), but that chest is not guaranteed to
  show up. Just play it safe and don't open anything you see in the
  southeast.
3. In the Nalbina Dungeons, do not open any of the treasure chests that
  appear in the room where you get the map for the area and regain your
  equipment (Confiscatory).
4. At the eastern-most section of the Phon Coast (Vaddu Strand), do not open
  any of the 16 chests that are there. This step is irrelevant, because if
  you are going to follow the walkthrough in this guide, you will obtain
  the spear way before you ever reach the Phon Coast.