The Gladiator used to be a monster that fought for the Galra Empire against prisoners of the Galra. He was supposed to be the opponent of Matt Holt when Shiro saved him by injuring him. The Gladiator was undefeated, but Shiro managed to defeat him by recognizing that the weakness of the weapon was after it's third strike.
Shiro managed to defeat the Gladiator and earned the nickname "Champion" as a result. Shiro learns all of this from Xi, one of the rescued Galra prisoners. Following Myzax's defeat and presumed death, his mind was put into a the body of a Robeast by Haggar who sent it to kill the Paladins on Arus. It's arrival coincided with Pidge and Shiro attaneoting to find th location of Matt and Samuel Holt in a downed Galra ship, a search they were forced to abandon when Myzax's giant pond landed, and crushed the wreck. Myzax then emerged, and attacked. Realising that a lion alone could not take down the Robeast, the team formed Voltron and engaged it. During the battle, Shiro discovered that the Robeast was the gladiator he slew in the pits, and used this knowlage to gain the upper hand on Myzax. Keith was then able to form the sword, cleaving through Myzax's body, making the Robeast explode, killing it. Haggar was furious with her creations death, and promised to make a stronger beast to claim Voltron.
Myzax had a large staff, that could generate an orb at will. The orb was directed using the staff, and is visibly indestructible. There is also a good chance that the orb is made of quintessance.
The main weakness of the staff (which reappears later when Myzax is a large robot) is that after the third use, it must recharge. This is the time to strike, because Myzax has little defense past his staff. After Shiro learned this, he took advantage of the flaw in the design to strike the monster, defeating it with his simple weapon.
This upgraded version of the robot is much more reliant on defense, and is built to stop attacks, even from the massive robot Voltron. Because of this, the robot can take multiple full - force punches from Voltron, even sustaining damage from his special attacks in order to retaliate with his orb. Even Voltron's shield cannot stop the orb full - force.