The petitioner is... Migelo! Well not directly. You accept the hunt by bartender in Sandsea in Rabanastre, but the reason you're asked to kill the enemy is to make a special wine, and it's on the commission of Migelo. Buyer personal details are restricted, but just go to Migelo's Sundries after you have taken care of the monster and talk to Migelo. He'll tell you that he has ordered a very special wine ;)
About 51 000 HP
Resistant to Libra and Dark,
Vulnerable to Water
Marilith is a great snake, which can be found in Zertinan Caverns by large breaches in the cavern's wall. You won't find him immediately, and it won't appear that fast. Kill everything near these breaches, stand by them, and wait for a while.
His attack is virtually only Tail Spear, which may cause slow on the attacked character, besides he can combine ordinary attacks in long combos, so the fact that he has only one attack doesn't mean that you can underestimate him.
The easiest way to get rid of him fast is to use 2x berserk and a healer. He should also beside healing cast Haste on everyone suffering form slow. First make him cast it on himself and then on others.
Bounty: 2200 gil, Serpent Eye (1440g), Teleport Stone x3