The weak opponent(s), known as the Underdog(s), tend to languish in two ways:
(1) from slow speed (usually earlier levels),
OR (2) a terrible driving accuracy (usually higher levels).
To complete the Underdog Challenge, the Underdog is:
(1) helped to finish in front of everyone, including the human player,
OR (2) allowed to finish in second only to the human player, first among the other opponents.
The Underdog(s) must be exceedingly weak compared to the stronger opponents. Levels with readily-accessible Out zones are excluded unless the Underdog does not get Out too often.
Ultimate Underdogs are the AI team with the weakest overall performance throughout most of the levels of each locale.
Easter Island: Xu
Hong Kong: none