Aps
Aps is probably one of the strangest bosses because one of his attacks will also do damage to him. Sewer Tsunami is a multidirectional attack, meaning that it can either come from behind the boss or from behind the party. Whomever it comes at from behind will get more damage. Aps ends up doing more damage to itself then you will cause to it from this.
Aps has a fire weakness so be sure to hammer him with your Fire materia. Keep the party's health up and this battle should go by quickly. If you want to silence Aps try using Cloud's Cross-Edge limit break as it has a good chance of paralyzing this boss for most of the battle.