You MUST level your treasure hunter skill for your pet dragon ASAP and you MUST dig every treasures in every fight.
I'm on hard with my paladin, just got to Montero, and I have over 200K in cash, and I buy every set piece and costly gear I want. I dig 3 treasures every fights.
And level your resurrection skill ASAP. I never buy any units anymore.
Some tricks:
-use Royal Thorns and spam Plants like there is no tomorrow, and then bring those summons on the front row of fighting for decoys, even the archer plants.
-use Cyclops + Target spell to draw fire from Archer units.
-spam bless and heal in every fight like there is no tomorrow to get the medal to raise Inquisitor, Paladin and Priest damage ASAP.