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.
iRobot Sumo vacuuming robots, which are creatively changed in order to fight on the ring.