In an earlier thread, I asked the same question and they basically said they can't figure out how to make them just work for the player only.
What you can do is use the "heal all" whenever you're on the brink of death - it heals everyone, but that still beats dying. I only tried the "kill all" once, while I was at full health, and my health went to 0, but I was still alive even though the others died, so I used a regen item to restore my health and was good to go.
Alternately, you can play tournament mode and use the cheat codes (they don't work in the skirmish for some stupid reason). I think some of the cheat codes also work for everyone, but the "all skills", "level 20", and "teleport" work on the player only. Just using teleport alone can give you an enormous advantage.
But I also found that once you get a handle on how to play a particular demigod, the game is pretty easy, when played at normal difficulty. Even "hard" isn't so hard. The trick is that the demigods all play pretty differently, and what works for one may not work for another. I started with Rook, but Unclean Beast was the easiest for me to master. Then I tried Oak and he seemed incredib;y weak, until I got the hang of him, and suddenly he seemed way overpowered. In the end, they all feel powerful once you learn them.
The other thing to understand is that killing ("farming" the reinforcements is the key to leveling up, and leveling up faster than your opponents is the key to winning. In the early game, killing one wave of reinforcements can be worth almost a levels worth of experience.