This is why the Curse-ExtremeSpeed Combo is never going to earn a STAB...

[Wiped]

It's unorthodox, but a Dragonite with Focus Punch could get the job done.

Turn 1: Kangaskhan Curses; Dragonite uses Substitute
Turn 2: Dragonite tightens its focus, Kangaskhan uses Extremespeed; The substitute is destroyed. Dragonite uses a super-effective Focus Punch for a massive OHKO.