The Kulaman Ruins Battle.

The Giant Devouring Quicksand Battle.

The Nian Oasis Struggle.

The Tulu Desert City Siege.

Kang Oh participated in all of the battles between the 2nd and 3rd princes as a mercenary.

He also fought most of the Empire Guild's eight squads. 

Kang Oh fought Moby Dick and Taker again in the Nian Oasis Struggle, but he wasn't able to kill them there either.

He encountered Ridgeley, the captain of the Brown Bear Squad, in the Kulaman Ruins.

Kang Oh had given him a humiliating defeat in the past, so upon seeing him, Ridgely rushed at him without any regard for anyone or anything else. 

Not only was he unable to pay him back, but he was humiliated even further.

He was so excited that he wasn't able to fight properly. Once Demon Sword Akanhoff broke his halberd, the fight was over.

Indeed, he was humiliated once more.

In the Desert City of Tulu and the Giant Devouring Quicksand, Kang Oh encountered the captains of Black Wolf, Giant Soldier, and Yellow Dragon: Black Wolf, Tainos, and Eric.

Eric was his most difficult opponent. 

The Yellow Dragon Squad was the Empire Guild's strongest squad. The Spirit Swordsman, Eric, managed to bring Kang Oh to death's door.

If he'd killed Eric any later, then he would've died. 

The only captains he didn't get a chance to fight...