Well... keep in mind that realistic battles would have to take into account a ton of factors. Stuff like the arms and armour of either army and fortifications.
Duel system could be improved by modifiers, I.E. Generalship of the leaders, morale, and force ratios.
I.E. +1-25 defense per pt. of general, +1-25 something else per pt. of morale, and +1 damage per attack per person involved in the conflict, with the maximum being equal to the size of the other person.
Damage Modifier extra details:
A dragon, size 20, fighting an army of peon humans, size (
. Each peon will get +20 damage from having his friends try to stab the dragon all at the same time.
Two human armies would both get +8... This would increase the lethality of the duels and speed things up.
Arguments could be made that only the larger army should get the bonus, but I say that when large armies clash, it's bloody on both sides. Maybe reduce the effect by the amount you are outnumbered? If there's 100 on one side and 200 on the other, the 100 side takes a -1 penalty to their damage bonus? If you really wanted to cripple it, you only get the +Y for every X times greater than the enemy you are. (so the 200 side would get a +Y damage for the first duel, at least).
Could also be a 'retreat' option from the duels. After one party takes a yellow (or worse) wound, they try to flee the next round. The winner gets a parting shot, and after that, that duel is over and the loser is marked as a casualty, and out for the rest of the fight.
You'd see a lot more war wounded that way.
The way to keep the Megabeasts alive through this process is have them retreat more often. Also, their generalship skill should be crazy high. Since they usual lead themselves, they will always be their own commander, and will quickly legendary in the skill. This is balanced, because it gives the megabeasts a levelable skill that most of the stonecarving, soapmaking, pump operating, flattering, bookkeeping populace lacks.