on/at the level
Could you give some examples using full sentences? I don’t know what “body on/at the national level” means.
“At the national level” is often used in respect to competing in sports: to compete at [the] national level means that you are competing with people from all over the country, as opposed to local level which might mean competing with people in your city or surrounding area.
“On the national level” is similar, it means on the level which concerns the whole country, as opposed to local, or international level. The choice of preposition mainly depends on context, I would think, and they may be interchangeable in some cases.