Against humans and elves, the ballista is immediately useless. You need Grog or Nimus. Instead of the ballista, dark magic would be a good option (but it depends on your luck with spells, if you're lucky at all...).
1) Level up attack - frenzy, devilish strike, spell of speed (you can sometimes use tactics too).
2) Don't forget about opening the gates - hellfire is great!!!
3) Defense - fiery retribution.
4, etc. - it depends on what you get. Ideally, you'd want logistics, but most likely you'll choose dark magic with spellcasting. You'll be especially lucky if you get unholy word.
In general, I can say that these two races are the most annoying when fighting demons. Especially all their lucar.
I won't say much about necromancers, as I haven't played against them much. But I know that they are also annoying, so you need to counter summoning magic. It's desirable to have spell of speed and surprise attack, as necromancers' initiative is a bit lacking. Otherwise, I can't say anything specific.
If you have any more details, let me know.