He has to make his own shield and test it in combat against at least 2 opponents. How he makes the shield and what he makes the shield of is entirely up to him. Nobody said it had to be effective.

As for the ability Joshua will be able to prevent damage from (1) larger attack or (3) smaller attacks. Or he can mitigate damage, so people only get a little hurt from either. It'll be able to take more of a beating this way, but that's probably situational. If it shatters, then it'll cause him minor damage, but that's only if it takes too much damage too quickly.