With outside half Lee Evans penning Cyw back with some big kicks, it was Cwmllynfell who opened the scoring when wing Dylan Lougher crossed to give his side a 5-0 lead at the break.
The Cockerels stormed back after the interval and a cross kick from outside half Mike Huckridge sent centre Gareth Howells over for a try converted by Huckridge.
Once again the Neath valley outfit re-took the lead with a Lougher penalty and it was not until 15 minutes from time that Cyw went ahead for the second time.
Zac O'Driscoll stormed through and laid off to Huckridge. He stretched out for the try which he then converted to make it 14-8 in the Cockerels favour.
But Cwmllynfell were not out of it and wing John James put them back in front with a try under the posts which was added to by Lougher to give them a narrow 15-14 lead.
Four minutes were left on the clock when a clearance kick was charged down and hooker Luke O'Driscoll pounced on the loose ball to send Cyw to the Millennium Stadium.
In the other SWALEC Plate semi-final One East sides Rhydyfelin and Ystrad Rhondda met at Taffs Well.
Ystrad Rhondda were favourites to make it to the Millennium Stadium having lost only two league games all season, but Rhydyfelin had inflicted one of those defeats and they went into the match in good heart.
Rhydyfelin skipper Keiron Phillips was the hero with a hat-trick of tries in a 17-3 victory leaving Ystrad Rhondda with heartbreak for a second consecutive year at the semi-final stage.
The SWALEC Bowl final on 4 May at the Millennium Stadium will be contested by Wattstown of SWALEC Four South East and Fishguard & Goodwick of Four West.
Wattstown held off a spirited Blaenavon outfit to win their semi-final 16-15 at Pandy Park, home of Cross Keys.
Wattstown are top of their division while Blaenavon have only suffered one defeat in Four East so it was a battle of two in-form sides.
The Four South East club came through narrowly thanks to touchdowns from Chris Davies and Mike Tinson, with Tinson also adding two penalties.
It was also close in the second semi-final where Fishguard & Goodwick just pipped Cilfynydd 22-20 at Loughor RFC after a strong second half recovery from the Four East side.
Cilfynydd are top of their division but they were slow out of the blocks and forced to play catch-up during the second period when they put their opponents under a great deal of pressure.
But the Seagulls held out to make it through to finals day at the Millennium Stadium with tries from Simon James, Ed Bendell and James Griffiths.