Restart until you start near a Rocket Launcher AND near Sadako, your opponent. To get a time under 0:50, you need a kill in about 3-4 seconds. In case you're going for something like 0:55, 5-6 is fine. Watch your radar to see where she respawns and try to take her out right after she becomes vulnerable. Repeat this eight more times and hope for fast spawns.