The robots used in this competition are called sumobots. The engineering challenges are for the robot to find its opponent (usually accomplished with infrared or ultra-sonic sensors) and to push it out of the flat arena. A robot should also avoid leaving the arena, usually by means of a light sensor that detects the edge.
For different fight tactics the most common "weapon" used in a sumobot competition is an angled blade at the front of the robot, false flag for disorienting opponent robot, ladles or wheels with backward rotation. There is also Lego Mindstorms sumo robots, in which EV3 or NXT robots compete.
Sumo robots are built from scratch, from kits or from Lego components, particularly the Lego Mindstorms sets. The center of gravity in the design of the robot should be located evenly and as low as possible, this will give the robot greater stability and better grip with the ring during the battle.