hey i have gotten zodiark, what you need to do is have all the other espersincluding ultima, then talk to the GEOMANCER in jahara and he will then unloka the door to the second area of the henne mines where you can go through to reach zodiark.
By the way big warning, the enemies in that second section of henne mines are probably the most dangerous in the game, i went back recently for my bestiary, and was at level 99 with maximillion armour and stuff and was still healing a lot. lol so go prepared with buffs and good armour.

plus zodiark is no pushover, tip: make sure you have a strong non elemnetal spell like scathe scourge, for when he puts up his paling when he is almost dead, i only had bio the first time i went for him and spent about two hours running around him trying to finish him off before he darkja'd me and managed to ko my last three party members *ouch* lol