Use pillz wisely. Study cards. Give me all your gold.
Some other tips - prefer characters with high power. Play more characters from the same clan - for possibility of getting bonus of this clan. Prefer characters from clans, which have bonus for power or attack (Bangers or Ulu Watu for 2 power, Junkz and Sentinels for 8 attack, Montana, Sakrohm and Uppers for reducing opponent attack) - have better chance to win duel.
And level characters as fast as possible, characters are much better where leveled.