Pete... you said:

""...use science to dispute science.." so what do you think should be used to demonstrate that a theory lacks scientific validity."

What a strange phrasing. It sounds like you are verifying that some who accept Creationism do begin with the premise that certain scientific theories 'lack validity', and only wish to cast doubt on them.
The proper concept is 'test'. Science, using proper methods and logic, *IS* the way to test, analyze and evaluate scientific theories. I have no difficulty with that. What I am criticizing is the amazing idea that it is ok to deny, condemn and dispute certain standard scientific theories as 'unproven', and then to turn around and pretend to defend those actions by claiming scientific reasons!
How can I put it very simply.... IF you understand and employ the scientific method properly, you do not find the contradictions and flaws asserted by Creationists! They are ignoring widely demonstrated and proven measurements and data and then using careless, misinterpreted and just plain silly *bad* science to dispute the science they are against because of their religious beliefs... which they assert do not NEED scientific verification.
   I find myself struggling to express simply the flawed, convoluted argument form that "creation science" is forced to use in order to oppose the vast majority of standard theories about topics like evolution! It involves several logical fallacies, and one of the most egregious is 'circular argument'. It can take long explanations to show how they can even deceive themselves by using words in non-standard ways and inserting unwarranted assumptions to appear to dispute a theory, when they are actually fooling themselves with circular reasoning

I sometimes use a quotation to show a short example of what some of this form of thinking looks like.
An old woman is quoted as saying: "Of course it was a just war, my son died in it!"
There are various unstated but assumed premises in such a remark.... but it might be very hard to talk the woman out of her feeling because it is so emotionally important to her to believe things about her son, her country and some 'other country'. She ASSUMES her son would not be involved in anything unjust, then adapts her reasoning to fit.


Think about it.