That's what I'd be going for. Having established that one can breathe and eat through it, we know that gas and attacks that look innocuous will bypass the shield. I bet if one were to do that, he'd be claiming that the shield did in fact work against gas attacks.If it's the barrier I recall, it's permeable to gasses. It explicitly says in the write-up that you won't suffocate while encircled by it. If he's taken the advantage that lets ally attacks through then allowing him to eat is probably permitted.
My first thought would be to use a Flash attack (blinding) against him or a gas attack, both of which could be strong enough to KO the character depending on his other attributes.