If you are using a chain whip while trying to turn the freewheel off the hub then you've been trying to literally lift yourself off the ground by your own bootstraps. On a freewheel the whole core is one piece. It's not like with a freehub and cassette where there's that outer retaining nut that can be turned off separetly.

I'd second the tool in a bench vise and then just turn the whole rim counter clockwise to break it loose.