If it's an actual CIWS, it can't be used offensively. Lasers, Gauss, Railguns, or Particle Beams should all work fine though. Missiles will also work fine if you don't mind the cost of the missile and irradiating the asteroid.
No actual railguns... irradiating the uninhabitable asteroid isn't a concern... As for the missile cost, they are an obsolete design. I do have some that are 1, 4, & 9 damage though. Do I have better chance to kill a ground unit using a salvo of 9 1 damage missiles, 9 salvo's of 1 damage each, 9 9 damage, etc
I've been trying a variety of things while waiting for responses, but haven't noticed a clear winner... which is making me think that the chance to destroy one is per salvo and not increased based on the damage that salvo does