Let us use the trees
After seeing the title, I'm sure you all are thinking of Elves at the moment. However, it's applicable to every race!
When you play in Mirkwood, you'll notice that a number of the trees are rather large. This is fortunate for you because such large trees make excellent hiding places! Simply select one of your heroes, click on the trunk of the tree, and watch the hero disappear from view.
This trick allows even heroes that don't normally have a stealth ability to launch an ambush against the enemy.