For those of you who hate video guides, load up the first chapter single player on easy. you will come to A market, you can kill 5 enemies here in less then ten seconds. Just pump shots Into them and don't be afraid to run to them, its on easy after all. If you don't get it here advance to the area where you hop a wall and cars pull up with enemies. Even easier to get it there. Good luck.
AND I DONT KNOW WHAT this other guy is talking about with regaurds to 90% health, thats not a requirement.