Stop flushing, stop fuckin about with. New growth looks fine, what ever the damage, it was done a while ago. You will not bring those injured leaves back, looks like some sort of burn to me though. Leave her be and keep an eye on the new growth, not the old. In a week or so ya plant is gonna look like one scraggy fucker as the damage really starts to show. Just ignore it and carry on, ya will be fine.