You don't need to get enemy skills in order to get Master Command materia if that's what you're aiming for.

Anyway, you need to use manipulate on the beach plug in order to get it. It will never use big guard on its own, and they will most definitely not cast it on you, since it's a protective spell and all. Same thing with Zemzelett and White Wind, Zemzelett will never cast white wind on you, so you have to manipulate it. Beach plugs are easily manipulated, so once you know what to do, it should be a breeze!