Need to ask tenant to leave

Possession - Section 8 and Section 21

Hi I have a tenant who is causing a problem to the other tenants and freeholders in the block. They have taken out a police restraining order on her and they want me to ask her to leave. I was going to give a section 21 with 6 months notice to her, however I need advice because she suffers from depression and told me she tried to commit suicide last year. She appears to have mental health issues, she%E2%80%99s 70 years old and lives alone. She%E2%80%99s rented my flat for four years and believes the other tenants are harassing her. I need to make sure I do this correctly so she can%E2%80%99t sue me etc. There is no longer an option to let her stay, I%E2%80%99ve held off for a year to see if she%E2%80%99d improve.