Re: What does it mean: ’You can’t even dent the mustard’?
It's not an expression I have heard before, but I suspect that it is an extension of the saying that "someone [or something] doesn't cut the mustard"; meaning that the person or object is not capable of carrying out the task in question. So Slim is telling Preston that, not only is he not able to cut the mustard, he's not even capable of making a dent in it.
There's a similar dismissive expression in Br English, saying "You can't even fight your way out of a paper bag".
I'm not a teacher of English, but I have spoken it for (almost) all of my life....