Hit Y and go to Hero. From here hit RB to go to class. If you have finished the quests for a kingdom, their class will be able to be selected. I used Khetar for my gold star achievement so I chose Necromancer for my class. To check how many wins you have, click on your active class and hit RB to Perks. Under your perks to choose from, it will list how many victories.
NOTE: if it is showing you have more than 250 victories, simply play another battle with your hero and at the end of the battle the achievement will unlock. This happened for me.