All the races have at least one type of structure or person who can heal others. For example, the Humans have the Priests, the Orcs have the Witch Doc

One thing that some people do is hold back on the hero abilities. That is the worst thing you can do, so always make sure that you are using the most of the abilities you have at your finger tips!

Also, you might want to put them into something that might be extremely useful later in the level, except, perhaps, the most powerful skill that any hero can achieve, or ultimate skill, at level 6.

The others have three levels of effectiveness, but are all useful in one way or another. Spend them wisely... how you use your hero can change the course of the game.

Healing Units- A Must Have

All the races have at least one type of structure or person who can heal others. For example, the Humans have the Priests, the Orcs have the Witch Doctor, the Nightelves have their Keeper of the Grove/ Druid of the Claw/Moonwells and the Undead have the Blight/Statues. Also, some units have the ability to hell themselves by burrowing, turning to stone, etc, at an accelerated rate, and all units slowly heal themselves.

These units/structures are a must have, as they will heal any troops they see, especially when set on auto cast (right clicking the icon so there is a little gold sparkle around it) and can easily change the direction of any battle.

Gain High Elven Technology

Have you ever wondered why the High Elves are in the game, yet you never are allowed to use them? Well, in one Undead mission, you may. In Chapter 5 the "Fall of Silvermoon" where you are finally able to kill Sylvanas and end up turning her into a banshee, you may make a bansee from your temple of the damned and possess a villager; afterwards, you're allowed to build kick ass towers that can be built anywhere on anything, including cliffs, bridges, walls, trees, and even on water, though it has to be right near the shore. in addition, they're not that expensive and are some of the best defensive structures in the game, and the dragon hawk unit isn't too bad either. (I would recommend activating the immortality cheat to get the villager, though it is unnecessary)