Many (including me) have complained in the forums here about the battlefield-deployment bugs for a long time, but devs refuse to fix it, blaming it to be too various and complicated to fix. So let's just focus on the most important one, the one responsible for loosing this battle: the best support-units (with highest ranged-attack) are not always deployed in the second row (very easy to fix). And since the devs lately started addressing complaints with "is a feature, not a bug" (quote regarding the unrealistic peace restriction I complained lately), and this issue might also be declared to be intentional (for "balance" reasons) and be left unfixed, I will complain now from a different perspective: realism.
So let's look now at this specific battle, and compare it to a similar famous battle in history around that time: Battle of Issus (even locations are nearby). In the real battle, Alexander wins a decisive victory, even though he was heavily outnumbered, because of maybe two factors:
1. better soldiers/general (simulated in the game by unit stats, totaling to an overall Combat Power number);
2. "location[..] where Darius could not take advantage of his superiority in numbers" (simulated in the game by terrain frontage);
In the battle I lost, I also was outnumbered by 112 unit to 73 units, and I also had:
1. more experienced soldiers (same 1-1 general, my general defect was not active in Plain) as in total I had 644 CP against enemy's 532 CP (but the difference in XP is even higher if you consider only the troops participating in battle; for instance only my 45 elephants+archers+HCav had 507 CP, as my second army only had week LCav);
2. Terrain was Plain with 14 Frontage (10+4Cav/0 Defence) which should have only helped me against the bigger enemy army.

Instead, the battlefield-deployment does not use archers (+3 support bonus) as my support unit, but much weaker support-units (with +1 support bonus), therefore lowering my damage a lot during ranged phase (enemy will not be exhausted so much by my much weaker ranged attack) and by 2*14=24 less damage done to the enemy in melee phase. This is HUGE difference compared with having my archers as support-units as it should have been (they never participated in the battle), loosing an otherwise impossible-to-loose battle.



If you still do not understand the problem, let me simplify it for you: two full-frontage identical armies with units+cav in first row and archers in second (nothing more) will have 50%-50% win chance, as it should be. If you try to make your army more powerful (to increase your chances to win) by adding non-archers to your army, because of this bug you actually make it MUCH WORSE and your chances to win fall towards 0! This is completely unrealistic!
The devs punish players who make large armies. If not fixed, this will result in only one outcome: countries will never have huge armies, as these will always be defeated by smaller (still full-frontage) armies (of same units, same XP, same general), on ANY terrain (only difference: is smaller).
This is completely unrealistic, as in history (up to modern times) wars were usually decided in large battle between all enemy forces, not many small battles between many small forces.
Just look at how this is already the case in battles between experienced players:

Look how all forces are split by both players into small armies, not held together in a huge army, like in history. This is 20th century battle front, with armies stretched across huge front-lines, like in Word War 1 and later. This looks nothing like the Persian wars or any war of that era.
Again, all this is very easy to fix: just always put the best support-units (with highest ranged-attack) in the second row.
What do you guys think?