I really like Dugald. He has a very good starting skillset. If I manage to, I develop him to level 21 (usually the game ends at level 20-21 for me) like this:

Skilled Counterattack - Experienced Tactician Skilled Offense - Archery - Battle Fury - Retribution Skilled Defense Skilled Leadership Skilled Light Magic - Protection - Holy Vengeance Developed Logistics

Regarding creatures. I think it's not advantageous to use them for humans - Retribution is better. Unless the opponent is a Necromancer, that might be an exception. Humans' ballista isn't very strong. After all, its damage calculation takes into account knowledge and attack, and humans develop their knowledge slowly. The ballista is only good for Orcs and the Demoness Deleb.