The CMS Desk uses more than just the /admin folder, it uses a lot of other resources (found in CMSScripts, and CMSPages, etc).
First step would probably to use Cloudflare only for specific folders (like the /media folder, /CMSScripts/Custom, or etc) so only static content is cached, this is the majority of site slowdown is image, js, and css downloading.
I would try to leverage Kentico's built in caching to handle the page HTML rendering and not use Cloud Flare for it if it can be helped.
If you absolutely must cache HTTP content, then you'll need to really look at all the exceptions.