Worked some on the battle of the Arius River. Here is what I've got at somewhere around a 5:1 ratio.

Baktrian 1st Army

1. Somatophylakes Strategou (40)
same
same
2. Hellenikoi Kataphraktoi (100)
same
3. Mada Asabara (100)
same
same
same
4. Mada Nizhak Asabara (100)
same
same
same
same
5. Peltastai (160)
same
6. Shuban Fradakshana (160)
same
7. Kavakaza Kofyaren (200)
same

Baktrian 2nd Army (AI)

8. Baktrioi Hippeis (100)
same
same
9. Harauvatish Asabara (100)
same
same
same
same
10. Artish Pada (240)
same

Seleukid 1st Army
A. Somatophylakes Stratgeou (40)
B. Hetairoi (100)
same
same
C. Peltastai (160)
same
same
same
D. Artish Pada (240)
same
E. Sphendonetai (120)
same
F. Thanvare Payahdag (160)
G. Misthophoroi Peltastai (160)
same
H. Verkhana Kofyaren (200)
same
I. Thraikioi Hippeis (100)
J. Mada Nizhak Asabara (100)
same

Seleukid 2nd Army
K. Thraikioi Prodromoi (100)
same
L. Mada Asabara (100)
same
M. Thureophoroi (162)
same
same
N. Harauvatish Asabara (100)
same
O. Peltastai (160)
same
P. Akontistai (240)
same


And here is what I've got sketched out as a map:


The Seleukid force made it across the river (at a location the Baktrians weren't expecting) by dawn, and met a lighter force, then the main Baktrian army came and things were touch and go, till the Seleukid reinforcements showed up and they won the day. This is mostly a cavalry battle, with some light infantry thrown in - Seleukids specifically had 2000 "hetairoi" and 10,000 "peltasts" (which I take to mean some different types of light troops like peltasts, not all 10,000 Peltastai). The Baktrians have 10,000 horse total and I've added some light infantry also.

Is something like this plausible? The terrain should be relatively flat, from the Aria province. A double river crossing would be best (the way the game usually has most fords, but not all). Any thoughts? The account of the battle comes from Polybius 10.49 almost entirely.