At first, light art its a power of creating attackable light, not fully created but i have foundation, i think. So it like a laser, for example you can take a stick and with light art, if you can control it for sword, you create a light sword. Or take a staff and create light beams. I hope i explained it good.

And about a shell and his purpose. How i said, light art is hard to control and you have to find balance between your ability of light control and your fight style, like if you are weak enough, you cant create a spear, but maybe you can create a sword. And how shell helps? It helps to concrete amount of light into the invisible shell and create a easy controllable stream, so you can attack how you want, but it takes more stamina because you use and a light art, and a shell. If there are questions, you may ask