Very very simple, Unholy Frenzy on Lady Sylvanas during the fight, the damage done to her (30%) of hp, counts towards your dps. On top of it, gives the healers something to heal. Works on all modes.
Proof: Unholy Death Knight rankings for Galakras - Siege of Orgrimmar - World of Logs Check the logs and look at all their damage done. Top is Unholy Frenzy.