The connecting record
shownDigimon Frontier · 2002
Susanoomon killed Lucemon in Digimon Frontier.
in Shadowlord Mode, after the ten spirits combine
What each of them is ranked on
The killer · Susanoomon · rank 7
Ten warriors, one body — frontier
Frontier spends a season establishing that the ten Legendary Warriors are the Digital World's original defence, then has all ten become one thing to deal with Lucemon. Its final act is not a blow but an administrative one: it deletes the Digital World and writes it again, which is the only successful reboot on this page.
shownDigimon Frontier · 2003
Formed from all ten Legendary Warrior spirits, defeats Lucemon and reformats the Digital World.
The victim · Lucemon · rank 2
Officially a Rookie — frontier
This is the entry the page exists for. Lucemon's official level is Rookie — the same rung as the small creatures the heroes start with — and he once governed the entire Digital World, fell, was sealed in the Dark Area, and in Frontier requires the combined spirits of all ten Legendary Warriors to put down. The Reference Book has never corrected the level, because the level was never a measurement.
statedDigimon Reference Book · Lucemon entry
Lucemon's level is given as Rookie, and as one of the Seven Great Demon Lords.
shownDigimon Frontier · 2002
Final antagonist; defeated only after the ten Legendary Warriors' spirits combine into Susanoomon.
What this does not settle
One death in one work. It is the only place the record puts these two in the same room, and it is evidence about that meeting and nothing wider — not a general matchup, not a rematch, and not a ranking of its own.
The ladder above it is argued from everything either of them has been shown doing; this page is argued from one citation.
Where the two disagree, the disagreement is the point, and it stays visible rather than being resolved by whichever of them happens to be on this page.