It is possible that in polymorphing him you actually *did* create a Super Hobgoblin! I believe the framework was laid in version 1.4, and tweaked in 1.5, so that very randomly you would run into a monster that was MUCH tougher than the standard beast of that breed. If that occurred as deep as you were in the dungeon (via that polymorph) I would bet the monster's AC is so low that you will never hit it with a +7 weapon...
Further speculation (I am on a business trip so I cannot look up the equation right now)- I think you'd stand a chance with a +25 or so "to-hit" on your weapon, but if you are still alive, run!
