I disagree with gun control, but this isn't the best argument. Nothing can ever be eliminated, only diminished. You can't say that their gun laws are useless and should be eliminated, because of this one instance, anymore than someone can say that illegal immigration laws are useless because illegals still enter the country.
My view of gun control is simple, and never changes: I don't care what criminals do, because criminals will always find away around law, but don't punish me by reducing my right to defend myself to the best of my ability, by whatever means I believe are best.
This is a good argument, but the other one isn't a bad reply to those who say the countries who tightly restrict guns just don't have these mass shootings.
Well. Yes. They do. It's useful to point out that while Australia has fewer mass shootings than we do, they don't have zero because determined criminals will find a way, as you say. And the former argument is actually needed to prove yours because yours depends on "criminals will always find away [sic] around the law."
The other side is pretty much saying it flat out doesn't happen. The Australian style gun control fans in the US keep saying that since they Australia did their buyback, there have been zero mass shootings, which is actually not true even not counting this one.