Mount & Blade: Warband

Tournaments held in the Khergit territory (purple) often make you spawn with a javelin. Knocking people out in these tournaments counts towards the achievement. They also give you a decent amount of javelins to throw. Enemies on horses are the easiest targets as they often run into walls so you can keep pelting them in the back. If you're in a hurry just make a save before one of these tourneys and hope you keep spawning with one.

a quick way to get this is,
by forcing your army to retreat or to hold ground when you're up against looters/raiders, so you can pick them off at ease. it will take a while though...