You can win by putting Sonia very low on Health before the end of the 2nd round, very rare to happen unless you manage to counter a combo.
You can also win on the 3rd round if:
one of two options:
1) You either have more current endurance than your opponent and enough health
2) Your Power and Agility combined are very high.
I would say, the easy way to win versus Sonia would be to build a boxer with very high stamina, almost zero power and win by defending.
If you want to be bold, just make a very high action fighter with max power and agility, almost zero stamina, and survive until the 3rd round for a surprise k.o.