Wouldn't say there was a 100% trick to beating Tonfun but there is a decent zone-type strategy that will help you win 75% of the time when you face him for the first time when going for Invincible or Defeat Tonfun. Despite his name Tonfun is not what I would call a Ton of Fun to face. Terrible pun joke aside, here's my strategy.TonFun (Stage 10)
What I like to do is at the start of the fight against Tonfun I back up to the left a little and hit Down+Punch a few times. Tonfu will make his way to you. You want to make sure you're backed up just far enough to where when he nears, you jump over him to the right side of the screen. He will scurry backwards to the right and then to the left as you land. As he does this you will hit Down/Away+Kick(only move you need against him) and you should get a hit off of him as he retreats. Now immediately jump to the left of the screen and the right once again and repeat you offense. Tonfun will keep scurrying back and forth and fall for it over and over. You only attack as you land on the right side of the screen because he may introduce a tonfun to your face if you don't. Do not bother attacking when you land on the left side.
This strategy takes a few attempts to get just right but you will see what I mean after a while, it's all about proper zoning. I was able to get 2 Perfects on Tonfun using this method while going for Invincible multiple times only to have Blues ruin my streak. Once you get the right zoning in mind it works like a charm! Don't get discouraged, it takes a few tries to get it right.