A couple things I want to respond to right away, the rest I'd like to discuss a bit.
This seems to be what Elennsar wants. The PCs by themselves with no tactics have quite a large chance of dying and tactics are vitally important.
50% as a "large" chance kind of rules out the heroes ever being in a truly disadvantageous situation.
If you make it harder to win by playing heroically than by standing there hitting then it is stupid. Heroism isn't the safe option but it is a reasonable option (e.g. 75%).
One thing that has to be noted on this.
Sometimes, being heroic is the only option for success here and now, and succeeding here and now is kind of important in any situation.
Lots of heroic stories and stories of heroes have heroes that are outnumbered or at a large disadvantage. Since having a large disadvantage will make the game unfun, being outnumbered is better.
This I disagree on. If you find succeeding fun and hate to lose (period, full stop, etc.), then yes. If that's the case, I'm really not sure you'll enjoy playing a game where there are times you are outmatched.
Whether that's tragedy or just the fact there are bigger people than you are involved in this too (though how much that last is true depends, it is something I want true to some extent), the PCs are not the biggest individuals other than the Final Boss or something like that.
They're among the biggest individuals, but there are NPCs who can beat them.
One way to do this would be to use a bell curve system and to not use many character stats. The character progression would be mostly gaining new tactics. This also has the benefits of making characters fast to generate mechanically, since you could have toughness being poor, average or good.
Definately using a bell curve system (3d6 plus modifiers), not sure about new tactics - there are only so many tactics as distinct moves (dodging to the left is not really different than dodging to the right).
Other ways to change the probabilities of winning are hitting troops from behind (requiring facing), hitting one type of troop with another (this could lead to wargame type system rather than RPG), terrain and weather (requiring a whole lot more work) and morale (this requires a whole lot more to track).
I know I want to include morale and facing, and probably terrain and weather to some extent - at least so far as to make "bad weather is a problem." and such.
I am not exactly sure what I want to do in regards to morale - but something like this is probably roughly accurate:
From highest to lowest.
Enflamed
Normal
(Declining state, not sure what term to use)
Faltering
Routed
Naturally, if there is such a thing, PCs are subject to it (though they may not feel the same level of fear from a given thing as an ordinary-drafted-from-the-farms, they too can be demoralized.)
Trust in the Emperor, but always check your ammunition.