Is it the bolt head itself to loosen it or the threads on the engine where the bolt screws into the engine block?
If its the bolt head and you cant get it off then you can try hammer on a slightly smaller sized socket onto the nut head and if your lucky it will come off that way. I did that with mine when the head got worn and the spanner kept slipping then replaced it with a new drain bolt. Dont overtighten the new one.