The octopus would only stand a chance if he could get behind the narwhal unawares and squeeze him.
With the narwhal facing the octopus, the pointy bit (which is substantially long and strong), would be an ever-present threat.
If I was a narwhal, I would dummy the octopus to the left and switch right with lightning speed, running him through before his tentacles could grab me.
I have just spent five valuable minutes writing this in a Manchester library with the clock ticking down all the while.