Pause Frequently
There's a pause function for a reason. With the game paused you can teleport your allies, summon new soldiers, and cast spells. It can be overwhelming; don't let it be.
Teleport is Your Friend
You can't always have all your troops in the fight, but you can teleport injured soldiers off the front line and replace them with fresh ones. Do this frequently and you'll save a lot of gold.
Upgrade, Upgrade, Upgrade
While it's tempting to just keep your original warriors because you've invested so much money into them, be sure to switch them out over time for better, more expensive troops. They may not start out as awesome, but in the long run they'll be worth it.