The win over Scarlets sets up an intriguing derby final for Saints where they will tackle Leicester Tigers for the first silverware of 2012.
In a combination which worked to perfection, the experience of older heads like Tom Wood and Stephen Myler proved crucial in setting a platform for younger players such as Armstrong and Waller to show their worth at the knockout stage of a competition.
With the Gardens bathed in sunshine and a final against Leicester as the prize for the winners, both sides opened the game in positive fashion and only James Craig's try-saving tackle on the line prevented the Scarlets from taking an early lead.
The visitors looked sure to open the scoring on a number of occasions as they scythed through the Saints defence but some huge hits, in particular from Paul Diggin, kept the Welsh side at bay in a fast and furious first 10 minutes.
As a signal of their intent to progress to the final, Saints used their first penalty in Scarlets territory to kick to the corner and put pressure on the visitors. It was a decision that was to prove well-judged as Saints powered towards the line from the resulting lineout and Alex Waller crashed over for the opening score.
The try was just what the doctor ordered for the Saints who had started on the back foot and the ever-reliable Stephens of Myler and Jones exchanged penalties as the game continued at a rapid pace, giving supporters of both teams plenty to cheer about.
Scarlets had run well from deep in the opening exchanges but mistakes with ball in hand were beginning to give the Saints a platform on which to take the game by the scruff of the neck. It was one of those errors that proved costly as Myler punished the visitors with a beautifully converted effort at goal.
The mistakes from the Welshmen were becoming infectious as Saints conceded a soft penalty for offside straight from the restart to the frustration of Jim Mallinder and his coaching staff. Jones made no mistake with the kick and he reduced the gap to seven points as the game hung in the balance after an enthralling first 40 minutes.
A 10-minute half-time break had evidently done nothing to rid the game of the errors that affected the first half and Jones hauled the Scarlets back to within four points following side entry by the Saints as the hosts inflicted all the damage on themselves.
As annoyed as Mallinder would have been with the previous ill-discipline, so he would have been delighted with his team's response. Excellent work from Vasily Artemyev kept the ball in play on the right-hand touchline and Saints span the ball from right to left looking desperately for the opening. Quick ball was delivered superbly by Martin Roberts and Armstrong was the man over, the young winger diving over the line in the left corner to give the home side their second score.
Another wonderful covering tackle from Diggin stopped Scarlets from hitting back straight away but the full back was powerless to prevent Jones from reducing the arrears with another penalty goal as the visitors refused to lie down, despite being knocked with a few heavy blows.
Myler's third penalty was a deserved reward for the Saints after forcing Scarlets into more handling errors and his fourth just minutes later looked set to kill off the visitors' stubborn resistance as the gap between the teams stretched out to 12 points in the final quarter of the game.
Scarlets were in danger of being kicked out of the competition in every sense and as they made efforts to find the score which would get them back into the encounter, they were met with a green, black and gold wall at every turn as a professional performance from the Saints kept them out.
Despite movement on the scoreboard slowing down in the final 10 minutes, the action on the field certainly wasn't letting up. James Downey and Scarlets' Dominic Day were both shown yellow in separate incidents before Ryan Lamb slotted through his first penalty to complete the scoring and send the Saints to next weekend's final.
REFEREE Dean RICHARDS
16mins SAINTS Try Waller Con Myler 7-0
21mins SAINTS Pen Myler 10-0
23mins SCARLETS Pen S.Jones 10-3
35mins SAINTS Pen Myler 13-3
36mins SCARLETS Pen S.Jones 13-6
HALF TIME SAINTS 13 SCARLETS 6
43mins SCARLETS Pen S.Jones 13-9
48mins SAINTS Try Armstrong 18-9
53mins SCARLETS Pen S.Jones 18-12
59mins SAINTS Pen Myler 21-12
62mins SAINTS Pen Myler 24-12
74mins SAINTS Yellow Card Downey
76mins SCARLETS Yellow Card Day
77mins SAINTS Pen Lamb 27-12
FULL TIME SAINTS 27 SCARLETS 12