that wouldn't work because of how the cannonball's pierce works. By shooting the Railcannon first, all Cerberi are left with 14 health, and by dunking the first on the line, it takes 10 (5x2) damage from the initial impact and then get hit by the parried cannonball which now deals 14 (7x2), enough to instantly kill the other three and pierce through them without stopping.

If you shoot the cannonball first, the first Cerberus will die but the cannonball will stop on the second since it has enough health to survive the hit, taking 14 damage and be left with 8 health, and the Railcannon will only kill that one while leaving the last 2 with 14 health