The URL rewrite module starts checking after the initial trailing slash, so the following would remove the first level folder "pages":
<rule name="Redirect rule for aliased Pages">
<match url="^pages/(.*)" />
<action type="Redirect" url="{R:1}" />
</rule>
Testing with:
- http://example.com/pages/about-us/ :: 301: http://example.com/about-us/
- http://example.com/pages/advertising/ :: 301: http://example.com/advertising/
As expected.
Note that if you have the rewrite rules in a separate file, you will need to recycle the app pool or restart IIS for the changes to work.
Also, order of the rules is important too.