How to protect app IPA from hacks if reverse engineering is possible

There's always a risk involved. Even if you don't introduce vulnerabilities yourself, the platform may allow for exploits which in the end may offer an entry point for a malicious attacker.

As to your question: It is not safe to assume that a hardcoded URL, even if obfuscated beyond belief, can't be peeled out of your product. Always design your apps such that safety of user data is guaranteed (as far as possible) even if built in ressources get compromised. If the knowledge of that URL alone poses a security threat, then your whole approach and your clients API is inherently insecure. Remember that such information could possibly be captured by a man-in-the-middle attack (and other modes of attack) as well.

Avoid security by obscurity. Store sensitive data only on disk if it is necessary. As a rule don't allow PIN / TAN storage.

Some thoughts which may (or may not) convince your client that your app is as safe as it can be:

• As long as the app runs on a non-jailbroken device, it is unlikely that an attacker, even with knowledge of your apps internals is able to get to any user data, because the iPhone normally doesn't offer opportunities to interfer with your app
• If the attacker is able to get to your users data, and provided you have been protecting that data with all means available under iOS (-> keychain -> crypto chip ->...), then it's not your fault. It means the device is either jailbroken or there are vulnerabilities to the system itself which have been exploited, you just can't do anything about either possibility.
• It is impossible to prevent reverse engineering of your app. Even if you had put more effort into obfuscation, an attacker with strong motivation would still be able to get what he wants. Your client needs to get used to this as it's a fact.
• Other platforms suffer from similar vulnerabilities, yet on the iPhone at least you have a somewhat closed environment and a reduced risk of being attacked with trojans and the like.
• The governments and security firms get hacked on a regular basis, although they should now how to protect themselves. This means life is inherently insecure, cope with it.

I was recently researching over this and I found this article helpful, especially the part quoted:

The code for a native app is stored in the form of a binary executable file, which is further encrypted; its decryption is performed only when the executable file is loaded by the processor into the random access memory and the whole decryption process happens at the hardware level. That is why it is very hard to create offline decryption tools. The only way to decrypt encrypted binary data is on a jailbroken device with a few special tools installed.

Security in iOS: Protecting .ipa File Content by Stoyan Stoyanov