Well, a tricky question. Ideally, i always try to defeat the Senate using two stacks, that way i maximize my chances.
The Senate army is chock full of heavy cavalry, heavy infantry and some missile troops and is relatively balanced and experienced. My suggestion for a single stack pre-marian would be along the lines of:
1-2 Generals (ideally one for each wing)
4 of your best cavalry (mercenary heavy cavalry from the steppes is also a good idea)
9 unis of Princeps- ideally with at least 3 bronze chevrons
4 units of Creten archers
1 unit of onagers (to pick off some enemy heavy cavalry and break up formations of heavy infantry.)
This would roughly match the Senate army for firepower, but their heavy cavalry is the biggest issue. A unit or two of Mercenary Hoplites might also be beneficial to help watch the flanks.
This is just my recommendation that fits my playing style and my esteemed colleagues would probably have much better stack recommendations.
Good Luck!
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