[ToiletOperator]

I'm working on an old 335XI which needed new rear upper strut mounts. I believe these are original (150,000 miles!), and as such are corroded on there pretty good. Trying the regular way, 16mm with 7mm counter hold (KWV3) opened up my 7mm wrench, no good. Tried an impact, didn't get a great bite on the nut due to the low clearance and ZIP!, the socket tilted and stripped her pretty good.

So now here I am with a stripped nut, no counter hold, and the fear of making it worse.

Any ideas before I break out the die grinder and start cutting the tabs and washer? Pics below

Edit: I should add that the nut soaked in PB blaster for 24 hours before starting. no heat.
I'm using vise grips and thick rubber to counter hold on the unused portion of the shaft, but even putting two pairs on the shaft still spins. Putting 3 pairs on the nut strips more. . .
If I end up trying to cut the washer, I hope the thru hole is big enough the clear the nut.

[The Nightman]

I've always found it easier to break the torque on strut mounts with the suspension loaded, but I'm not sure if that's going to help in this situation

[Jaronbwall]

Oh my god. Knock on wood this doesn't happen when I order my suspension kit. Looks like you're busting out the torch and cutting stuff.

[KhaosKid]

Not sure if this is too late, but if this happens to anyone else this is what worked for me.

EDIT: your picture brought back a distant memory. I used the biggest vice grips I had (the name brand Irwin ones, cheap ones MIGHT not have sharp enough teeth to get enough bite on the shock piston to stop it from slipping). Then i got my buddy to hold the vice grip while I loosened the nut on top, or I just loosened it while the vice grips got caught on the underbody.

Either way, it worked. And this should work if you can get a good grip on the 16mm nut either with an extractor bit, or hammering on a 15mm socket or the closest smaller Imperial sized socket (likely need deep-well socket).
Use a 1/2” ratchet or breaker bar to get it off and you should get it off without much hassle. Don’t use an impact here, it will probably put down too much torque and cause the vice grips grip on the piston to slip.

Just make sure the vice grips are TIGHT. Also, this will likely scratch the shock piston, so grip it as high up as possible, or ideally replace the shocks.