My opinion is you need to do a few things:
1, Keep a good distance from him and use range attacks
2. eventually hell charge u, when he does run into a komidogu
3. Since that weakens him use ur strongest branching kombo
4.NOTE: watch out for fireballs and wind coming from him and try not to get grabbed, if you do fun to a Komidogu to make up for it.
My opinion is the best character to do it with is Noob-Smoke

You may not like Jade, but I think Onaga is easiest when you are her. Just touch one of the kamidogus and do her longest combo from her first stance. That takes out almost half his health.

I can't wait for Armageddon so I can kick Onaga into some spikes/grinders/trains/acid pits and see how HE likes it! (I don't use cheat devices.)

HAHAHA!! No shit, dude! I didn't think of that 'til i read this. Yea, i'm gonna send his ass into the spikes with a Bike Kick or Stone Column from Jinko, that stupid bastard. The first time i get his dumb ass in the air i ain't gonna EVER let him touch ground again! In fact, i'm gonna use Onaga to beat the piss out of Onaga, just for the irony!!
~peace!

i know a really good strategy with scorpion!
keep pressing sqare in the hapkido stance, then keep doing the kombo that goes like this: triangle,triangle,l1,square,x,x and l1. (if you have a ps2) hope this helps.

no duh he does double damage all you have to do is bicycle kicks and that inflicts 20-30 damage all by itself