Collocations for showdown - noun

climactic, final
The scene was set for the final showdown.

dramatic, ultimate
long-awaited, looming
military, nuclear
Commons, political, Senate
courtroom, High Court, Supreme Court
championship, quarter-final, semi-final, title, etc. (sports)
relegation, winner-takes-all (both BrE,sports)

have
avert, avoid
face, head for
force, seek
He was now strong enough to force a showdown with them.

showdown against
the showdown against Holland in April

showdown between
the title showdown between Brazil and France

showdown over
The government is heading for a showdown over the new proposals.

showdown with
the semi-final showdown with Australia last month

it comes to a showdown (= a showdown happens)
Of course I'll support you if it comes to a showdown.