One idea.

The challenge starts with a performance, either telling everyone how great you are, how useless your opponent is, dance off or another form of intimidation. The human will probably be at a disadvantage here because they're unlikely to know what orcs consider cool.

Then the combat begins. It's fought with wooden sticks covered in paint. Each hunting party carries these if they think they might enter a neighbouring territory. If the human hasn't then that's a big no no but luckily someone in the orc party has a set.
The aim is to hit your opponent in the leg (to show they can't run away). Then the arm (to show they've been disarmed) and then the head (the killing blow).
Getting the order wrong is another no no, but the orcs will happily explain if asked the rules and definitely explain if it looks like something is going wrong.

How you win or lose will have a large effect on future relations with a tribe. If you lost, but put on a good performance, got a few hits in and generally showed you're a good sport then you can expect favourable trading, possibly even a portion of the kill.
If you win, but said the wrong thing at the start, threw sand in your opponents eyes and just beat your them into the ground with no respect for tradition then expect a generally hostile reception.