FROM WIKI: https://dungeondefenders.fandom.com/wiki/Temple_of_Water
If you are re-running Survival to only farm Fairies, start on Wave 23 and ignore building anything at the middle-north core. That crystal will not become vulnerable until you reach Wave 25 (when everyone obtains a Fairy).
Fairy (Wave 25)
Seahorse (Wave 30)
Sea Turtle on a Treadmill (Wave 35)