Most of the basics have been covered already: there are enemies which work best in specific settings (pig cops for urban locations for example).
The supply/monster ratio should be balanced, there's should enough room to fight, smaller rooms shouldn't be spammed with large numbers of enemies etc.
Furthermore, I think good enemy placement simply requires a lot of in-game testing. There's a lot of variety in Duke's enemies, but the AI is very limited.
So sometimes an idea simply doesn't work, and sometimes it works even better than you hoped for.
Sometimes you need to switch to a different type of enemy, sometimes the environment needs to be tweaked.
Small example: when using battlelords as turrets they should be able to reach you with their mortars (making the fight more challening).
And it should be possible to avoid their hitscan attack, to keep things fair.
I think I succeeded in creating such a fight in the first Shaky Grounds, when 2 stayput battlelords appear on the other side of the cliff.
Because they're on a higher platform, they can throw their mortars over the cliff, making them very dangerous.
But there's still plenty of room to move around and to take cover, which keeps things fair (IMO of course
![:dukecigar:](https://forums.duke4.net/public/style_emoticons/default/wink.gif)
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Again, it's just a small example. Think of a combat scenario you would like to implement and test it to see if it actually works.