It won't flat spin with lots of throw, especially on the ailerons. But it takes some finessing on all the controls to get it right, it's not a slam 'em to the corners and hang on plane. I was doing that, couldn't get a flat spin and Jeff Williams got me to reduce the throws on my low rates and then he had it doing climbing flat spins.

This is just my take on the matter, lots of guys are flying Sledges, maybe some of them will offer their flat spin tips.