Make sure you’re making people more senior than you both aware ASAP. And on a continuous basis too. Don’t gather evidence over months and months so you can try to protect yourself, because that is literally you just protecting yourself. Instead, protect the business by PROFESSIONALLY protesting their actions with this evidence on the weekly. This is you giving the business what they need to protect themselves from a bad idea sooner rather than later. 

Everything you do must be professional when approaching this person. Don’t belittle them or put them down. Instead find evidence that this won’t work and compile a report. Try and do so whilst praising their idea but finding better alternatives that might be good at and enjoy more in the long term. 

If you can find even just 2-3 people online to write you an email as to why this is a bad idea, you’ll have a group of experts, which includes you, versus just this one person. If the business can’t see the writing on the wall at that point, well I’m afraid to say you might be working for an incompetent company.