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