However, I do not believe that once a demon has gained an advantage over you, only another person can free you. If you have the Holy Ghost, it's power, Jesus' name, and His Word can be used to free yourself.
Say you're attacked by a spirit of fear. God's Spirit will let you know that the way you're feeling and these 'thought forms' aren't of you and they aren't of Him. So you say something like, "In Jesus' name I will not give into you. The Lord has not given me the spirit of fear but of power and of love and of a sound mind." And it will leave. The Bible says in James 4:7, "... Resist the devil and he will flee from you."