Challenging other heros is quite easy. Go to the challenge tab at the hero tab and select one of the shown heros to challenge. The heroes shown are usually in your range, but you can reload the selection, if you like. Once a week there is a tournament at the castles, the world championsship is in the middle of the world.
In order to be successful in the duels, you have to assign your skillpoints and select an apropriate class. Every class needs different skill, so chose your skillpoint allocation well.
You can perform 5 challenges per day. Use the extrafeatures, to do more duels, but notice, only the first 5 duels a day grant experience.