This achievement and the Servant of the Secret Fire achievement are relatively easy for 80 points each. It might take a little longer but you can finish the levels on the easy setting without ever using your heroes. Build up your fortress, build lots of defense towers and resource gathering buildings, and purchase all the unit upgrades. Build as many units as possible and apply all the upgrades to them, then head out and destroy. Make more units as you lose them.
The best way to keep you heroes out of harms way is to put them in towers or mines. You'll be able to give a command to your entire army and your heroes will stay put.
If heroes are on a horse or flying creature, they need to dismount before they can enter a tower of mine. There are a few heroes that won't go into towers or mines -- two flying creatures in the last level for example. You'll need to be careful with those and make sure they don't wander into trouble.
One other key is to save every so often. In addition to the "automatic" save system which prompts you to save at the end of each level, you can also do your own manual saves during a level. If you lose a hero you can just reload the latest save. This saves time compared to restarting the level.
When working on this achievement, be careful not to save at the end of a level unless none of your heroes died.