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.
Buy cheap cards at the market.
If you're lucky you will find cards for 100 or less.
Buy a Candy Jack => good card for very little money