How do I prepare this exterior concrete wall for paint?
If there is six or seven layers of paint over the concrete then it will be sufficiently thick to be noticeable if you attempt to just scrape a single area and paint.  It sounds as if a number of different people over the years have been putting off the dreaded inevitable conclusion that you should completely scrape and remove paint from the entire surface and repaint it with a proper exterior enamel approved for concrete.

When scraping paint always make sure to wear a dust mask as one of those layers may be lead based paint and it would be extraordinarily bad to breathe this in.  When scraping keep children, animals and pregnant women away as they are particularly susceptible to ill effects from lead exposure.

Most hardware stores sell lead testing kits that will let you know if any of this scraped paint dust contains lead.  A word of caution though that if you happen to verify and know that lead is in the paint then if you ever go to sell the place that you would be legally obliged to add this information to the sellers disclosure.  Sometimes it is better to just treat the paint dust as if it contains lead and not know for your own sake.