This should be done at the same time as the 25 laser mine kills. Play King of Time on mp1 (rooftops) and find the mines in the lower brick hallways. Take turns killing king with mines.
3 things worth mentioning -
1. sometimes mines fall through the floor or disappear so i suggest throwing TWO at the wall instead.
2. simply pick up the crown in the beginning, and every time the king is killed, the killer becomes king and waits to be killed next.
3. if you are going for king kills make sure to put the points to 76 or higher bc you get 3 points for each king kill and 1 point for the initial crown pickup