Here is the full list of all 33 GLADIUM achievements.
Defeat Aetius.
Defeat Aetius without rolling.
Defeat Thaddeus.
Defeat Thaddeus while he is stunned.
Defeat Othos.
Have Othos slay 3 of his own beasts before you defeat him.
Defeat the Alaricus.
Defeat Alaricus without letting him regain health.
Achieve your first victory in the Arena.
Kill three enemies with one blow.
Have a disarmed weapon kill an enemy
Kill two enemies while caught in a net.
Kill a thousand enemies.
Die a hundred times.
Have a thousand gold.
Get a victory in the pits.
Retire with 4 or more victories in a single pit fight.
Unlock Endless Mode.
Defeat a boss in Endless Mode.
Achieve a thousand points in Endless Mode.
Destroy all barrels in the Hub.
Defeat the Twin Champions by killing Brutus last.
Defeat the Twin Champions by killing Arrius last.
Achieve victory in the Arena without taking any damage.
Complete one contract.
Complete 50 contracts.
Complete 100 contracts.
Defeat Lucar.
Defeat Lucar with a Falx.
Kill four enemies with one attack.
Defeat Titos.
Defeat Titos using only a Gladius (No Blood Perks, Helmets or Shields).
Get 25 victories in the Pits.