I've been playing for a few weeks now, and as far as duels concerned. All I see are peacekeepers, berserkers and orochi's in duels. Just today, I played 10 games in duels and 9/10 I got partnered against one of these heroes. Only one time did I see a warden, and even then after checking his stats(after adding him to friends) his most played was Peacekeeper...
I would address this issue with the ban option, although we can fight against the assassin classes with other classes. It's not only harder, since they can change attack direction faster but also have deflects, status effects and unbreakable combos... < These three passives I think make them the most popular, and you either nerf them to the ground or add the option to ban them...
OR make other classes fight them on equal grounds...
I personaly would preffer the 3rd option, since I like assassins the way they are...