Is this grounds for Slander?

The girl i work with has lied about a million times about me and a lot of other people. She has told my fiance that i said he was ugly. She did that twice. NOT true. She has told my boss things that i didn't do or say. She told two people that my co-worker steals food and supplies every night from work. This all takes place at work. Everybody is fed up with her and I want to know what can be done legally. Her grandma is a resident there, so that is why my boss has done nothing to her. Please, somebody give me some good advice. I am desperate!