https://www.youtube.com/watch?v=2_tdLCFRaPY&t=152s
Basically, Lieutenant George (accidentally called to be the defense counsel, because Baldrick sent for the wrong person) tries to get Captain Darling, who is the prosecutor against Captain Blackadder, to be a defense witness.
Darling actually does have firsthand accounts of the events in question, he tried to phone Blackadder to tell him his orders from the general were to do something, but Blackadder claimed that there was static on the line.
Granted, I don't know of any reason why Darling would be qualified as a lawyer either.