admin case and libel against mistress

hi atty.,
I am planning to file an admin case and libel against my husbands mistress. I am still living with my husband while they still have relationship pero hindi po sila nagsasama. People are talking about them cause they seen them together in public places. The girl keeps on telling to other people na hiwalay na daw kami ng husband ko. My husband's relationship in his FB account is separated but all legal documents are still in married status.My husband and the girl is both working in a government office, is it possible to a file a case against the girl only? admin case para matanggal sa work at the same time matanggalan po ng license and libel dahil po sa issue na pinapakalat nya? what are evidences needed and other preparations po once i file the case? Do i need an atty when i file it at their office or civil service? How much will it cost?
Thank you and more power.

i do not believe you will require an attorney, but i am sure this will affect your husband also. not also clear if you have enough evidence to back your claims.