This is request for custom development as it requires a lot of time to be implemented.
Some tips that you should take into consideration:
Flexslider, when enabled, targets only one class -
.flexslider which is defined via
flexslider-custom.js. For multiple slider you might want to target different classes or at least know what you are targeting.
Now you need to get those post attachments and style them properly because flexslider requires special markup (most of the sliders require the same but just chunch of images won’t do it.) For basic idea from WordPress side you can look here. And here for Flexslider specific markup.
Now you should put it all together and style it properly.
Sorry that I can’t help you with complete code but I hope that you got at least some idea about this.