Near impossible to hit without completing Bridge Puzzle
Impossible to be harmed by when the Bridge raises
The Bass only appear at Akropolis Tower, Bridge. It is a mutated fish NMC. If the bridge puzzle is solved (by entering 561 on the keypad next to the bridge), they are of no consequence.
If the puzzle is not solved, Bass can deal a lot of damage to Aya (along with POISON status) as she walks through the water and they take passes at her. While not 'impossible' to attack the Bass, the extreme difficulty of aiming and attacking with a weapon in the water while being constantly hit may require a player to uses a large-scale, area-of-effect PE spell like Inferno or Apobiosis to take them out.
While Aya gets credit for three kills, there are six in the water. The player only needs to kill three to win (the rest disappear).
It is possible to retreat from the water, but the player will still be in active combat mode and leaving the room will incur the usual penalty.