Putting it together: I cannot assist regarding that.
"With" – possible replacements: "regarding", "concerning", "about". Or "in relation to" but maybe too long. Short alternatives like regarding.
Here’s the revised text with each word replaced by 3 alternatives in spintax format:
"That" – alternatives could be "it", "that", "something", "the matter". Hmm, "it" is straightforward. Maybe the matter? But "that" is the original, so perhaps include it as one of the options. So the matter.
That seems better. Now checking each substitution: