One thing about Carriages;
While the Magic Circle of the driver often determines a carriage's top speed, how strong the carriage is and how well it will do in collisions is completely dependent on its build and the driver takes no responsibility for its sturdiness.
As such, the fact that Caius was at the Fifth Circle and slammed into the Border Guard carriage with a Seventh or maybe even Eighth Circle Mage/Mage-Knight at the wheel, didn't mean he was at a disadvantage.
They had chosen this carriage for a reason; it was large, it was bulky and looked decrepit but it was a unit that would win in most collisions. And that was the case here.
*BANG!*
The Border Carriage was knocked out of the way and went careening off to the side where many of the border Guards who were on foot had to act quickly so they weren't crippled.
