Perth finally cast aside its easybeat tag this season and an eight-game losing streak to clinch a four-wicket win.
It came at the expense of Trevallyn, which spoiled the 400th club game for 47-year-old evergreen Andrew Fitz.
The bold performance was the side's first breakthrough for 322 days, dating back to the 2018-19 final round.
Things have changed since the Demons collected their fifth victory after just missing out on a semi-final berth in a dramatic three-way battle.
Cressy, who edged Perth out, has since joined Legana and Beauty Point exiting the Premier League that has streamlined a tougher new six-team competition.
Perth has found the going tough, losing to Hadspen on run rate in a rain-affected clash earlier and a three-run defeat to ACL to end the year.
The battles with Trevallyn included the opening match being abandoned after 36 overs and Perth falling 11 runs short of victory amid a last-wicket stand of 33 that ended with its captain John Hayes bowled out for 64.
Saturday's win was about more than just redemption.
Trevallyn set a challenging 9-189, all on the back of a recalled Daniel Kirk.
The opener hit 68 off 94 balls, but struggled to build a partnership until Clayde White hit a handy 21 off 36 for 58 runs for the fifth wicket.
The hosts crumbled to 8-136 until Liam Brown (27 off 16) and Caleb Lawson (22 off 34) added a further 53 runs.
Matthew Rigby removed key batters including Kirk to finish with 3-37 off 7.1 overs.
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Perth lost Hayes for five, but Matthew Lawrence (35 off 37) and Keelan Beattie (57 off 87) put on 78 for the second wicket to stay on track.
When Beattie was finally dismissed, Jake Smith all but guided the Demons to a worthy win with 27 in 31 balls.
Hadspen also reaffirmed its double finals chance off an empathic 10-wicket belting of Beaconsfield.
In the new year where the Swans have severed all ties with their offseason Sri Lankan quartet, it was the a pair from the island nation that tore their rivals apart.
Thanuka Dabare smashed 64 off 38 balls and Dilan Jayalath added 67 off 54 that included 12 fours and five sixes to win in just 14.2 overs.
Beaconsfield had set 8-141 following 50 from Drew Clark and 35 from Chris Walker.
The pair put on a vital 83 runs for the fourth wicket.
Chieftains captain-coach Liam Reynolds took three early wickets in 25 balls to leave the Swans reeling at 3-16 in the 10th over.
The match between ACL and Longford was called off at the 12:30pm cut-off point.
The umpires deemed the hard wicket too dangerous to start up play.
Overnight rain had soaked the University Oval on the arrival of the players.
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