I haven't tested how it works in Stronghold, but previously, an enchanted ballista would push the Hero back on the turn order scale. IMO, it's not worth focusing on machines, considering that you won't be able to develop them much when playing against a human opponent, and the skill itself rarely appears. It's better to level up more useful skills for the elf.
P.S.
If you really want to play as a Chaos Knight, it's better to max out the Warlock skill and cast Rain of Fire.