The Warriors scored 16 runs in the powerplay before stepping on the gas.
The Royals also got their top position back in the points table.
In their previous meeting, the Rangers won by seven wickets courtesy Cameron Delport's half-century.
Fletcher raced away to his maiden T20 ton off 53 balls.
Despite the loss, the Vikings are through to the playoffs.
It will be tough for the Pakhtoons to stop a determined and fearless Northern Warriors in the final.
It is tough to pick a winner out of these two exciting teams but Northern Warriors seem invincible at the moment.
Pakhtoons take the field as favourites due to their heavier batting line-up and should end on top against the Legends.
Pakhtoons are expected to edge past Maratha Arabians due to the consistency in both batting and bowling departments.
Afridi's team won with 15 balls to spare.