Outside of fighting Hedlok for fun in the Game Modes of VS Hedlok or Hedlok Scramble, there is only one known way of challenging Hedlok as a Boss of Arms. In order to face Boss Hedlok, perhaps the toughest fighter in the entire game, a player will have to challenge the Grand Prix at Level Difficulty of 4 or higher. However, that's just the first step in path to Hedlok.
A player will then have to make it through the gauntlet of this extreme difficulty, including defeating the Champion himself, Max Brass (or whoever the champion of the gauntlet is for that specific round). After making it through all of the competitors including the Commish, Hedlok will suddenly appear after the final battle. It will immediately latch onto Max Brass' body and challenge you to a Best-Two-Out-Of-Three Round Match. This is the only way to find and battle Boss Hedlok.