Numbers are the main factor, plain and simple. You wont be able to recreate a Battle of Crecy, where you're outnumbered 6-1 (if in fact those numbers are correct). You generally don't defeat armies twice the size of yours, as the AI is also pretty good about appointing it's own generals. Martial doesn't really matter for a field general. It gives a better a change of doing the right tactic in battle, so you're more likely to do a "massive longbow volley" with a 30 martial than 15. However, it's traits that really matter, and it's here that martial matters, as 20% percent defense bonus, factored into a martial of 20, gives something like 40% defense bonus. A general with a martial of 30 who is craven is worth less than a general with 20 and is brave. Really the only the you thing you want your highest martial character to do is be a Martial, and if he's a good general as well, perfect.
If I'm going into battle, I tend to take the highest martial skill that has the unyielding trait or other defensive traits, so right now a wing of my army has general with a martial of 23 with unyielding and patient, leaving my level 28 martial Martial with no traits at home to study technology. Of note, the army is led by my bastard son who has martial of 29, with unyielding, patient and brave.
