Some people were a little surprised by the difficulty of the Heroes V demo. Timing is crucial on some scenarios, so there is not time to walk around and look at the scenery, no matter how pretty it is. Below are a couple of tips from the forums, which include some minor spoilers:

• Inferno Campaign Map: Fabrice suggests getting speed bonus from items and buildings, ans skipping everything else. It's safe to say he knows what he is talking about.
  
• Haven Campaign Map: After rescuing Isabel, give her all your armies and wait for Agrael the the towm. ThunderTitan knows the location of the Tears of Asha if you're stuck afterwards.
  
• Standalone Map: There are various tips on how to complete the map here. Basically, find some backups and hit and run the enemy.

Do a little reading. There's still lots of time to refine your skills before the final game comes out.