Minecraft wiki entry for Creating_a_challenge_map, associated text: Create a 500*500 barrier cage. Position all the players. Have command blocks hooked to redstone clock to summon powerful monsters, such as vindicator with Sharpness V axe, zombie in full diamond armor, skeleton with Power V bow, charged creepers, etc. Summon an ender dragon or wither as well. Keep the map on adventure mode so people can't break the redstone.