With the 3.5 update the classes now track the active usage of the hero with that class equipped in battles. In order to include your hero in your troop you will need to equip a weapon. Which weapon you use does not impact the class leveling, but some classes do have affinities/bonuses for certain weapon types now.
Different battle types award different amounts of XP (on win) according to the patch notes:
Quests, Challenges & Explores give 1 XP to Champion Levels, all other battles (PVP, Arena & Events like Raid Boss) will give 2 XP
The actual amount of XP needed to go up each lvl is n+1 (current lvl + 1). So to go from 0 (class just unlocked) to 50 would require 1+2+3+...+50, which works out to n*(n+1)/2. Or in this case 1275 XP
to go from start to lvl 50.So depending on the type of activity you do (with a single class equipped), at best you are looking at 638 wins to go from scratch to lvl 50, at worst 1275.
Link to patch notes in case you want to learn more: https://gemsofwar.com/3-5-patch-notes/
*Edited to call out that these need to be wins to get XP, no participation trophies here
*From rocking23nf -
In case this doesn't pop for anyone, like me, I went into my hero tab and went to the champion level tab and then it popped.