1 Ryouga gets lost, either before or during the fight.
 2 Ryouga is wearing a watermelon (but check below).
 3 Ryouga gets lost and Kunou chases Ryouga
   off into the sunset (literally). spamfic anyone?
 4 Kunou gets a lucky strike with his stick on a
   vital spot. (its actually a bokutou I know...)
 5 Ryouga gets wet.
 6 Ryouga falls victim to the dreaded 'plot device'.
 7 Kunou benefits from a 'deus ex machina'.
 

Actually, there's another one: if this is either a set weapons-only fight
(like a kendo match) or Ryoga agrees to only use his umbrella.

It's happened before. At the begginning of "Tendo Family Christmas Scramble,"
where Ryoga and Kuno have a short fight that is strictly bokken vs umbrella.
Kuno holds his own, because, no matter what else you say about him, NOBODY
else in Ranma can match him for pure swordsmanship technique. 

-Mercutio
"NO!! I WANT IT!!! IT'S MINE!! MINE!!!!"