There are several optional parts of this final mission, none of which call for serious combat. Completing the optional parts of the quest gives some closure to Syanna's case.
Geralt and Regis pay a visit to the shoeshiner to find out once and for all who the her final victim was supposed to be. Pay a visit to the beggars to intercept the final letter. Once you have that, you can visit Syanna to confront her.
Once that conversation is completed, you'll be transported right to the medal ceremony for Geralt. You have the option to tell the duchess of Syanna's plans to kill her, or remain silent. After the ceremony, Duchess Anna asks you to stay and be present while she confronts her sister. In the end, you'll have the last say on Syanna's fate.
After the reunion, Geralt jumps to Regis' graveyard home. After a drink, Regis asks you to pick some mandrake roots. Use your witcher sense to find them, but be sure to use the gloves and mask Regis gave you to stay safe. After picking them, you'll find that Regis is gone. Use your witcher sense to track him down and find that he's been ambushed by bruxae. Take them down to continue the final dialog between Geralt and Regis.