This trophy is now a litter easier to obtain now than it was when the class was first introduced.
To unlock the Hero class, the player must complete a Client Order from Koffie. To complete this Client Order, the player must earn a Title which requires reaching Level 75 in any two non-Successor Classes.
Koffie is the girl you received all your level cap break CO's from.
There is still a long grind but you no longer need the 3 different classes reducing the time to earn this one quite a bit.
As per other class trophies the trigger is when you equip the new class at the class counter.