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Doesn't look like it's stripped too bad. If you can fit a 6 point socket in there it will probably work. Otherwise use a small pair of vice grips if possible.

 

Worst case you remove the head with manifold attached and deal with it off of the car. If you do that, put some of the other nuts back on so it is not hanging by one point.

But yes, 6 point socket. Try one slightly smaller than 12 mm such as 7/16 or 11 mm. Use a hammer to tap the socket on lightly if needed. Make sure it is all the way on the nut before trying to turn.

 

What mk378 said. Try and tap a 7/16 on it.

Otherwise get a vice grip on it.

Torches work wonders with seized nuts/bolts.

 

There is also stripped nut extractors that will bite into the marred heads of nuts and bolts.

They look like this:
Mastercraft 6-pc Bolt-Out Damaged Bolt/Nut Remover Set | Canadian Tire

 

This They sell similar kits at homedepot and harbor freight

 

Now if you could tell me the secret to get the remains of a seized o2 that has a broken spiral flute bolt extractor in it out without an acetylene torch. heh

 

I have had success with stuck bolts and Mapp Gas (yellow bottle)

 

Had this problem. Can't use a torch since the intake manifold is plastic. Although it's rated to be used in an engine so is presumably heat resistant. That nut is impossible to get to with real leverage. I used a TWENTY INCH air hammer chisel, held firmly with my opposing hand to aim it at the left side of the nut. It took several minutes but finally came off. Had this not worked, I would have continued until I split the nut and would have pried it off. This experience taught me that specialty nut splitters also exist.