Ideas needed to remove stubborn nut.

Hoping someone with more shop wisdom can chime in.

I am removing my sub-frame, and the weld-in nut casing broke off and now I have a square fastener spinning.  I have tried jamming all sorts of tools, bolts, wrenches in there to stop it from spinning.  No luck.

Do I have to re-weld that casing? Should I buy an air saw and cut the bolt out?  Any tool that will fit in there to hold it so I can zip it off with the impact?

The other three came off fine... grrrrrrrr 

Grind or drill the head off the bolt where your putting the socket on it from the outside and can get at it.

You'll need a new bolt for putting it back together tho and I'd put a regular nut on the new bolt so this doesnt happen again.

If you have enough access, you could weld that nut to the frame to prevent it from spinning. 

I ended up tack welding it and heating it up a little bit.  Used a breaker bar and the son of b came off!