Jiango is a beast kept in the Hellena Prison dungeon that the guards and wardens of the prison used to torture and kill prisoners. He is occasionally fed prisoners to keep him satisfied. Dart and company fight him after falling into a trap-door by the prison guards.
Jiango has 1280 health and the suggested party is Dart, Lavitz, and Haschel, all at least level 12. Jiango is an extremely powerful boss. He's also a huge earth attacker, as he seems to be obsessed with jumping up and down and smashing characters with falling rock, this can do over 100 damage on the characters, so it can be quite deadly. He also uses his breath to cast confusion on one ally, although it only hits 50% of the time. Physical attacks won't really do that much damage, but magic seems to be very effective in this fight, particularly wind spells. Make sure that Haschel attacks a lot though, because his Double Punch is very powerful. The real weapon against Jiango, though, is the Sachet you find in his chamber. With Jiango put to sleep, the battle gets MUCH easier.
Anyway, this battle doesn't really have much strategy to it, since Jiango is basically the same throughout the whole fight. If you have Rose instead of Haschel, it really won't be much different. She MUST get into Dragoon form quickly to use Astral Drain though. Lavitz should also get Dragoon form quickly so he can use Wing Blaster. These combined should do over 150 damage. Using Shana is unadvised as her health and physical armor are so low that she will almost constantly have to defend to make it through the battle.