The type of enemy you get depends on the amount of trophies you currently have. The levels of your Heroes and enemy Heroes do not affect matchmaking.
NOTE: If there are no suitable enemies for you at the moment, the selection pool will be expanded and you may face enemies who have more or less trophies than you. If this happens, the amount of trophies for a victory/defeat will change correspondingly — if you have more trophies, you will get less of them if you win, and vice versa.