For me the simulator gives a 4% chance on victory, which is confirmed by other simulators and feels about right.
Most of the battle is determined by the damage done by your xbows and the big bertha.
Lets say your attack attack takes 11 rounds then there will have been about 950 attacks made by your xbowman but only 17 or so attacks by the big berthas. So while you can easily assume that the damage done by the xbow attacks averages out, the amount of damage done by big berthas can and will vary per simulation and thus introducing a small chance to win.
Spoiler