Ask them their rank, they will answer something like : "Silver", say something like "That high ? I think you were ahead of us, but only Bronze !", then softtaunt them every time you can, you just kill them "HA ! I killed a silver, do you think I could deserve bronze in fact ?", and just try to tilt them, you wont win if they keep their mind sane, if the smurf loose his mind you have a small chance.
Thats how I win my last normal game yesturday,
(I have all the logs if someone is interested)