Go for Lucy style Vectors.

Low-vibration: Solid, able to physically attack and defend, but can also be swatted away.
Mid-Vibration: Semi-solid, can pass though skin and attack blood-vessels
High-Vibration: no-solid, can pass though anything, but has no physical substance to attack, and is invisible.

Over-use can cause damage to the body, and Vibration levels must be climbed. (So has to start low and then go Mid) Mid can't pass though armour, of anything the like.