I really like how you have built the game so far. The game is fun and challenging. It's normal that not everything works perfectly at the moment. I belive that nerfing cards isn't the solution but maybe there could be a way to limit summoners according to leagues, like only common, rare in bronze; common rare and epic in silver and gold; all levels in diamond. This would even out the playfield I believe.
Also, I believe it would be great if you added the rulesets no melée, no range or no magic. If one of these would be added in each game, this would require a new setup each time.