For two weeks each year, college students take over a massive stadium and fill it with fighting drones, plastic ammo, and rapt spectators. This is Robomasters, held in Shenzhen by DJI, the world's leading drone maker. In part two of Hello World Shenzhen, Bloomberg Businessweek’s Ashlee Vance goes inside the world of Nerf-style robo-warfare to spotlight the kinds of innovation that have kept DJI, at the forefront of a booming market, and demonstrates why you may not want him on your squad.