Without scripting, it looks like the best you could do is 8% critical hits.
Set the skill to "Physical Attack", and the actor to "Critical Bonus"
There is also a "Critical Bonus" on weapons, but skills don't use the weapon. (I don't think)
If you don't need to use the +4% "Critical Bonus" for actors, the current script could be modified so that option,
along with a "Physical" skill could produce a higher critical bonus....
This would allow you to apply the effect you want without having to modify the scripts. (Just using the RMVX Gui).
Check existing scripts, as Mouse suggested, or post in the Script Request & see if someone has time to do this for you.
Be Well