How to make more secured proxy in APIGEE?

[#1]

I want to know how we can make more secured proxy.

As I am finding in web that,

if client credentials are hacked ,then Authentication layer will be passed in Verify APIKey policy.

if client credentials are hacked or even the access token,then the OAuth Authorisation layer will also be passed.

So,how can we build a strong security layer?Does SSL really matter to solve this case?