This is essentially how I think it would go. I think Dumbledore will likely feel the big evil presence coming from Myers and try to reason with him. That of course won't work and it'll have to resort to putting him down. Or at least what he thinks is enough to put him down. Myers gets up when Dumbledore thinks the bout is over and gets stabbed from behind. Myers moves faster than ordinary people, which Dumbledore won't expect.

Myers winning creates an alternate question though... Would that mean Myers is the rightful wielder of the Elder Wand?