Have you ever heard of plasma in sci-fi movies? When you do, it is either as a weapon or a shield. However, the shield idea would not work as by the basic physics required to have the shield would not work as plasma would melt the ship that it protects.

However, plasma would be able to be used as a weapon as long as you can keep it in a semi-fluid state of a wall-like structure. Thus by using anti-gravity generators to force the plasma into a ball shape around your enemies' ships, you could melt the ship, as there are few things that could withstand the type of heat generated by the plasma. It is sweet that think of what you can do with something like that!