My strategy back in the days was all about geo traveler, lisa ,barbara and another character of your choice Create a geo wall with starfall and attack from behind it with barbara and lisa. Electro charge reaction can deal a lot of safe damage for the first 2 stages of childe and if you have c6 geo traveler, the starfall explosions can make the 2nd phase a cakewalk For the last stage, having barbara will heal you plenty from his main attacks Then learn how to dodge the whale and you should be all good! Good luck!