|| Head down the sewer and proceed to the elevator leading to B2. Glenn will be waiting there. He'll give you the Water Tank B Key. Take the lift down to B2. Proceed to the southeast part of the area. Now head right down the stairs next to valve 6, then right again up the next set of stairs. Continue north and examine the flashing red light at the door to use the key. Defeat the boss and return to Glenn.
Boss Battle - Wraith Virus
First make sure you learn these skills: EP Regen, Focus I (or Focus II), EP Half & Hammer Coat. Now I suggest leaving both Wraith Feelers alone, and focus your attacks on the one at the center. Both feelers drain your HP & EP. It doesn't take away that much HP, so don't worry about that too much. As for your EP, you don't need to worry about that either, as long as you have the first 3 skills I mentioned above learned.
Cast Flame Sword on your party (along with Attacker, if possible) Stock up to full, and begin your offensive. Wraith Virus has a pretty nasty strike-type attack, but it won't get that many chances to use it if the feelers are still there. Once Wraith Virus is gone, just finish off both feelers to win this fight.