Defeating stage 6-9 where the child of light is the boss is rather difficult. I defeated them when I was at player level 57. pretty late right. I know. so I used a petite devil, lore weaver, big guy, prima donna ( of course who wouldn't ), and ethereal guide. now ethereal guide is a damage dealer so you can use any damage dealer you like but make sure it is ranged since damage dealers like demon slayer and shade just charge in and they die before you can cast a healing spell. so use tracker or ethereal guide or use a bombin goblin or black crow as your damage dealers. the incinerator is also very good. now use big guy or one tank so that once their ultimate recharges you can use it to distract all your enemies away from your other heroes. and time your petite devil stuns very carefully. use it only when one of your heroes is dying. I had all white grade heroes rank 7 except big guy ( grade blue ). thank you.