After you get the Sluice Gate Key from the White Mousse hunt, go to Garamsythe's Central Waterway Control and turn off all four gates. Now open #10 and #3 (lights on) and go to number 3 Cloaca and turn on the switch. Back in Central, close 10 and 3 and open 11 and 4. Go to No. 4 Cloaca and turn on that switch. Now go back to central and turn on # 3 and 4. Cuchulainn is in the No 1 Cloaca, and it's a tough battle. You should probably wait until you have Curaja and Reddas, and levels in the 40s.