to put it simply :
3 hit combo, sidestep, 3 hit combo, sidestep, 3 hit combo, etc etc
Hes the easiest boss ever - pathetic!

hes a lot like beating Moloch. super easy. only difference is that with Onaga ur also trying to get to the Kamidogu's.
ok every one has a combo thats really good to use against Onaga over and over. some r long some r short. for example with Sub-Zero the best one is to use his 3 styles branch combo over and over. thats a long one. with Liu Kang just go to Nunchaku and button smash Square to do a 4 hit combo over and over. thats a short one. every one has a combo that works well against Onaga just like in MKDA every one has a combo that works well to beat Moloch very easily. so instead of my listing all of them for each char how bout u just go experiment

make him giggle, laughter is the ultimate defense breaker

with baraka, sidestep->chop-chop blades (if all hits connect is a 4-hit combo...I think) (since you are alrady pressing down you can pull it off quickly). I beat onaga on medium (default) almost flawlessly the first round w/o use of the komidogus. but just make sure he doesnt grab you and do his tombstone piledriver move so just duck and weave....

Baraka- Chop Chop Blades
Liu Kang- Bicycle Kick
Shujinko- Bicycle Kick (unlock it)
Lie Mei- Her little flying punches
Kenshi- Telekinetic Flurry
Jade- Has a really good slash attack with her weapon
Dairou- Remember Kung Laos kick ass broadsword comboes well he has the exact same ones I found the B attack easy as hell to use (I have an Xbox)
Raiden- Nun Chuan good stance against him
Sindel- She has some good weapon attacs as well
Scorpion- Teleport Punch
Ermac- Has that kick ass Axe weapon use X or Square if you have a ps2 and back and he does a three hit combo
Nightwolf- Tae Kwon Doe use The B B B combo on Xbox or OOO on ps2 and hell do a good 3 combo

tow words.....LONGEST COMBO!

Set the # of rounds to be 1, and the round time to be like 20 seconds, then hit him once, and avoid him. Smash the Kamidogu if you need to get some more hits on him.