It seems to me that you can not be a true expert in all sports. Thus, if someone is confident in his results in this way, he is very much mistaken. It is always necessary to control and study only one sport, as well as all the teams and people who participate in this sport. It is then that you can make certain conclusions and predictions that make it possible to make the right bet, which will lead to good results.
It`s impossible to be an expert in all sports, professional gamblers are experts for specific leagues or teams, in individual sports for specific players. With knowing more about players and their capabilities it`s easier to make a bet, and with that chances for making a profit are higher, but it`s gambling, there are no guarantees.
In my experience, there is no "the easiest" profitable betting strategy. Every time we make a bet we are risking, risk and profit are determined by odds. Professionals consider so many factors before making a bet, with that, they have more chances for making a profit.