When you first encounter Gladiator he is invulnerable, so rather than attacking him directly, run around either clockwise or counter-clockwise to each of the four consoles in the corners of the center platform. Press X to activate the first console, then run to the next and press X again. Once you have done all 4, Gladiator will be subject to attack for a limited time. Just wail on him until his health bar changes from red, indicating that he is once again invulnerable. Then hit the consoles and attack him again until he is down.