What do u mean by harden?
If u mean harden the muscles do standard bodybuilding routines for ur abs e.g. crunches, v sit ups, weighted crunches, hanging leg raises, etc... www.bodybuilding.com
This will allow u to take a punch to the abdomen.
Hardening is impotant (hell its nice to be able to take a punch) but do u have stone eye balls Stone Carver?