11mm don't work then use a 10mm with a extension and hammer it in. it cannot be all that tight. be sure to use 12pt socket on it.

 

Screw using sockets. That often just rounds it off more and makes it worse Use the tool specifically designed for that job, the damaged nut remover. As you turn it, it actually cuts into the nut to tighten the grip and give you more torque. The smaller sized socket method doesn't usually work very well and is difficult.

 

nut splitter? lifesaver for me once.