See what the goal is, first of all.
I always try to represent the problem graphically. I just use the Visio to show where the problem is after doing some reaserch. In addition, it is helpful to bring some real life stories with similarities to that project. You just can't say "this will fail" or "this is wrong". Show the table or flowchart - it is always better accepted that verbally.

Your picture will be memorized by your boss, and hopefully will force to re-think his/her position.