I found this comment on stackoverflow, pixelfreak is the author. But, there are multiple places where it is said that you cannot set the -fno-objc-arc flag on multiple files at once in Xcode. Not true. You can read it from the source here.
But, here’s the deal. I, and doubtless many of you, use third party libraries like Sharekit, for example, that aren’t compatible with ARC yet. One solution is to set the -fno-objc-arc flag for the files that you don’t want using ARC. Sharekit has a ton of files so it would be garbage to have to go through one at a time.
The solution is to select multiple files, hit enter (not double-click), put in the -fno-objc-arc flag, hit enter again. You’ll see that all your selected files now have the flag set. I was glad to find this piece of information.