Never underestimate your opponent. A slightly unusual idea as to how Lu Ten might have died.
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Lu Ten perches on a wall surrounding an elaborate garden. He signals the Yu Yaan archer next to him, and volley of arrows knocks out the guards within.
Their goal is simple – kidnap Toph Bei Fong, forcing the girl's parents to surrender the city. Gaoling will surrender with no loss of Fire Nation troops, and Lu Ten can join his father at the siege of Ba Sing Se. Even if the girl is an earthbender, she's six years old and blind; how dangerous can she be?
The Fire Prince hops down from the wall, and the girl screams.
Toph isn't sure what's happening, but she knows she is in danger. She uses her earthbending to pull the stranger down, down, down, and runs back into the house – but she can still feel him there, trapped.
Lu Ten struggles for air, trying to fill his crushed lungs, but he can't breathe, he can't even draw on his firebending to end it quickly, and it hurts so much, and everything else is fading away, and, and …
Toph shudders as the underground heartbeat stops, leaving a harsh empty space echoing under the garden.
The stranger's vibrations were frightening, burning and flickering at the edges, like nothing she'd ever felt before. Now they're even scarier, as everything in the garden reverberates except his body …
Six years later, she feels a vibration so much like his, and she wonders if this is what it means to 'see a ghost'.
My explanation: It never actually says in the series that Lu Ten was at Ba Sing Se when he was killed, only that Iroh was there. In fact, a translation of the writing on the picture of Lu Ten we see in 'Tales of Ba Sing Se' implies that he was not at the siege. Toph got involved because Toph is cool, and her almost being kidnapped would help explain why her parents were so overprotective. I may or may not expand on this idea at some point.