Arniston grabbed their second win of the season and also their second consecutive victory over St Andrews at the weekend.
In a stirring cup tie Arniston were 1-0 up with less than a minute played.
A deflected pass meant for Liam McIntosh fell to Andy Watson who fired home from 25 yards over the head of United keeper Shaun Hunter for the perfect start.
Next a raging shot from Thomas Thomson was well saved by Hunter and over the first 15 minutes, Arniston’s strong start had seen their keeper Dale Cornet yet to be tested.
It gradually became another tight end-to-end contest and in the 18th minute it was 1-1 when a stunning rocket shot by United’s Ross Graham beat Cornet all ends up.
But four minutes later it was 2-1 to Arnie with an impressive finish by Jack Hay over keeper Hunter’s head.
Three stunning goals in the opening quarter of the tie had the crowd fully entertained.
Next a brave save by Cornet at the feet of United’s Falconer denied the Saints the equaliser.
Excellent Arniston play saw Liam McIntosh shine before in 36 minutes a double yellow over dissent directed at the referee saw United’s Michael Fleming red carded.
In a tough tussle, Arniston’s Jack Hay was booked in 40 minutes also for dissent before a well struck free kick by Scott Devlin was well saved by Hunter.
But despite being a man short in the second half, the Saints were full of determination as Arnie for their part looked for an elusive third conclusive goal.
An injury to Arnie’s Watson in 68 minutes saw home replaced by Danny McFadden.
In a nailbiting finish, Arniston held out to secure progress to round three of this cup.
Arniston: Cornet, Aitchison, Deland, Dunn, Woods, McTernan, Thomson, Devlin, Hay, Watson, McIntosh. Subs: Waugh, Fairgrieve Miller, McFadden, Fleming.