On the rear end that much rusted, you will fight every bolt, and need to replace nearly everything.
Try to simply unbolt (be f... careful not to drop it on your head) whole crossmember/subframe, and perform work off the car. Maybe even replace whole sub frame assembly from other Focus.
I'm just finishing my rebuild.
Blue SVT 2002
Our Lady of blessed acceleration
...that should be written as 'The First Law of Internet Forums'