The north Wales outfit ran in six tries in what was a comfortable 40-5 win.
Five of the tries came in the first 40 minutes, ap Myrddin securing his hat-trick with the only score of the second-half.
With just two places between the sides before kick-off, both teams were looking to put down an early maker.
Centre Johnny Wright missed a chance to give Penallta the lead when his penalty effort drifted wide of the posts.
And RGC made the most of the let off, soon taking the lead with a try from scrum-half Afon Bagshaw. Sustained pressure from the home side saw the ball spread left with the Penallta defence under pressure. Though they stemmed the initial tide, a try was inevitable and Bagshaw burst round the side of a ruck and dived over under the posts.
Fly-half Mitch McGahan added the conversion for a 7-0 lead.
Penallta responded with a fine finish in the corner from No. 8 Gavin Jones. Wright missed the conversion but from there on RGC dominated the rest of the half.
Prop Tom Hughes capitalised on a loose ball to score in the corner, before wingers ap Myrddin and Iolo Evans ran riot.
Ap Myrddin scored in the left hand corner before Evans brushed off some poor Penallta tackling to extend the RGC lead.
There was still time for a further try before the interval. With a number of men outside him, McGahan’s cross kick found Myrddin in acres of space, the left winger collecting the ball to score with ease.
McGahan converted all four scores as his side led 35-5 at half-time.
With the home side boasting such a comprehensive lead, both sides knew the game was up.
A number of changes from the two sides saw the flow of the game disrupted and though the contest remained fiercely competitive, the quality of the attacking play subsided.
There were few clear cut opportunities in a fairly listless second-half but there was time for ap Myrddin to seal his hat-trick, finishing off a move down the blind side to end a 40-5 win.
