Here is the full list of all 15 Gladiator Trainer achievements.
Sanctify the crescent blade
Complete the main quest
Visit the Royal College of War
Defeat Emeric the Champion
Defeated the Champion of Willowsfall
Defeated the Champion of Tonstead
Defeated the Champion of Wainton
Get married
Visit a horse in your stables
Defeated the Champion of Stonyford
Defeat the Champion of Waldley
Defeated the Champion of the Dunes
Defeated the Champion of Sandworth
Defeated the Champion of Greenwood
Accepted Folke's assignment