You can only alter the bombardment strengths of the bombardment units.
With version 1.16f, city population, city improvements, and tile improvements all have a bombardment defense of 8. Units have a defense equal to their defense strength. I BELIEVE defensive terrain bonuses are taken into account (10% for grassland/plains ... 100% for mountains).
When bombarding because of an activated ZOC, defense strength is DOUBLED (
my 'rant' theme of the day -- should not be doubled and there is nothing I can do about it in the editor 
)
Unfortunately, dramatically altering bombardment strengths for one thing may have yucky affects when used against something else (e.g., units / cities / tile improvements).
Note: Just in case you aren't aware of it yet, the Cruise Missle is the only bombardment unit which can 'destroy' a unit. Nothing else will remove the last hit point.
