Cuchulainn is located in the Garamsythe Waterway's No. 1 Cloaca under Rabanastere. To get there, complete the "Waterway Haunting" and "Lost In The Pudding" hunts to open the way to get the Sluice Gate Key. Once you have the key, go to Waterway Control and follow these steps to get to the Esper:

1. Begin by closing the No. 3 and No. 10 controls in the Central Waterway Control.
2. Enter the No. 3 Cloaca Spur and use the No. 1 South Waterway Control at the end.
3. Return to the Central Waterway control and open No. 3 and No. 10.
4. Close No. 4 and No. 11, then enter the No. 4 Cloaca Spur and use the No. 1 North Waterway control.
5. Return to the Central Waterway Control again and open No. 11 and close No. 3.
6. Enter the No. 1 Cloaca and find the Boss at the low point on the south end. Save the game before the battle.