strange that you only made it to the blue tent. When I took it, I was already more or less unafraid of both red and purple.
In general, there are two General tips:
1) Be sure to level up a mage with Expert Earth (I used Ingham for this role, due to his innate eagle eye). Give him one Archangel and send him after strong heroes. In one turn, you can wipe out ma-a-any creatures with Meteor Shower (and later with Implosion). Against red, I'd leave them with only naked Black Dragons this way.
2) As soon as you feel you can crush 4K Ice Elementals (virtually) without losses — rush like the wind, after gathering the cap. Even if you don't kill purple, it's better to trade Strongholds for Necropolises.

And at the end of the game, I ran into the problem of how to kill the Big Bad. When forces are equal, he dismantles me like a Lego set. Painful, isn't it!