Facing a enemy with a knife it's itself a dramatic situation! I've been throwgh situations where the agressor just wanted to intimidate me. In this case you should NOT REPLY. However, if He/she wants to injure you seriously ( or even kill you), try to take him/ to the ground, attacking his/her knees with your legs while defending your upper body with your arms. If, for any reason, you can srike his/her head, do it quickly and with no mercy at all! Remember: never start a fight, but if you get into one, finish it!