In my opinion , just like how u gain proficiencies in any class of weapon , by hitting x amount of times . i.e. 1000 for lvl1 , 2000 for lvl 2 , and so on
im thinking maybe every time you get hit u get 1 , so if you got hit 1,000 times you would gain lvl 1 proficiency for what ever class armor it is. and so on.
just a suggestion
think about it