I deflect two curses and fire off three of my own in return. He tries to cast a Killing Curse several times, but each time I distract him with my own spellwork. Two more curses whiz in my direction and I parry them both. Then I hit him with a Sectumsempra and watch with satisfaction as he slumps to the floor. I can't kill him, but the curse, as I know from personal experience, will do it in no time. When Severus told me in sixth year what spell Potter hit me with, I learned it and added it to my repertoire (true to the motto "our enemies teach us life's most valuable lesson') and it has proven useful many times since. Still, I disarm the bastard as an extra precaution. 