You’ll have to show she can stay with you ie landlord letter of approval. Send her an invite letter. Make it clear you will cover her costs and medical insurance.
Any correspondence or statements you’ve sent money to establish how long you’ve known her. They just don’t want her to be a burden on the state and go back when she says. List reasons she has to go back, job family etc
don’t use agents. I have had g/f visitor visas, spouse visa children visa and never used an agent and never refused. Do the homework and provide the documents and no problem.
They make money from visas they just want to make sure there is no burden on the state.