It's all backed up by recordings and emails, as well as prior testimony.
That's takes away most of the opportunity of the defence to undermine the case because Cohen is untrustworthy. The story of what happened is transparent and has sound support underlying it. There's little doubt it's against US election law, although it doesn't sound like anyone had the conversation about it.
The prosecution has done its job well. And there's no get-out-of-jail free card if Trump said he didn't know it was illegal. So this case looks pretty tight to me, with no special expertise in the matter.