To be able to face this esper you have to beat "White Mousse (rank V)" firs (details in "Ordinary hunt notices" part). Due to that you will obtain Sluice Gate Key, which allows you to manipulate with sluices in Garamsythe Waterway.
Sluice control devices 11 and 4
Sluice control devices 10 and 3
First, you have to close locks 4 and 11. After that you will be able to get to device No 1 North (left screen).
Then go back to the main room, open locks 11 and 4 and close 10 and 3. This will let you get to device No 1 South.
After opening north and south lock No 1 water level in a central part of sewer will decrease. You will fight there with Cúchulainn after that.
Main hall before drying
Main hall after drying
About 130 000 HP
Resistant to Libra
After getting through all those combinations with locks, go to the central point of sewer. Cúchulainn will appear when you go downstairs
Foobars try to disturb you during the battle because they support the esper. Also, your HP level will slowly decrease during the battle.
Cúchulainn doesn't use his force during fight. Actually, all his attacks blend something in your characters stats, and in conjunction with slow losing your HP, it becomes dangerous weapon. During that fight focus on healing yourself. You will always have time to defeat your opponent when your are alive. My proposition is, let the strongest character attack the esper, and the rest should heal him (and of course they can fight in their free time with Foobars). When jellies don't appear anymore, you can start attacking Cúchulainn with two characters.