How did Bumi lose to Aang in S1E5?

Bumi seems far more concerned with seeing what the Avatar can do in a fight than in actually defeating him. Sure, some of his rocks would have probably killed Aang if they'd hit him, but that's the rockbender way. No pain, no gain.

Note that the King is very clear that Aang has passed all of his tests, including the final one, "a duel". No-one said that it had to be to the death.

King Bumi: Typical airbender tactic: avoid and evade. I'd hoped the Avatar would be less predictable. Don't you have any surprises for me? Sooner or later, you'll have to strike back.

[Aang finally strikes back]

King Bumi: Hehe! Well done, Avatar. You fight with much fire in your heart. You've passed all my tests

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At the end of the episode we learn that Bumi is in fact one of Aang's friends, now older and substantially wiser. These tests weren't intended simply for the King's amusement, they were to train Aang and to get him ready for the difficulties ahead.

Sokka: Why did you do all this instead of just telling Aang who you were?

King Bumi: First of all, it's pretty fun messing with people, hehe, but I do have a reason. Aang, you have a difficult task ahead. The world has changed in the hundred years you've been gone. It's the duty of the Avatar to restore balance to the world by defeating Fire Lord Ozai. You have much to learn. You must master the four elements and confront the Fire Lord, and when you do, I hope you will think like a mad genius!

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