If you look at intraspecies fights over game, and often even interspecies fights, they are rarely to the death. Both parties want the prey for food or other use, neither wants to lose their strongest warrior over it because that's worse than losing some game. So, it would be reasonable proposal that this duel is to submission, not to death. The most honourable way to win is one that shows superior technical skill and physical strength, that is, is most difficult for human compared to an orc. Pinning or locking the orc in a grapple, for example. Dishonorable ways are those that are too obviously lethal as well as those that are seen as not requiring great skill or strength to carry out, such as poison. Likewise dishonourable are techniques that cause long term harm but aren't all that decisive for a duel - for example, a person only has two eyes to gouge out, but being blinded doesn't stop a skilled grappler from winning a single grapple.

Of course, this all changes depending on general alignment of the orcs. The above is Lawful or Neutral outlook for when the human is essentially considered "one of the orcs". For Chaotics, especially Chaotic Evil, there is no honor beyond winning, and no capacity for the losing party to retain face other than still being too strong for rest of their party to take down.