Two converted tries and two penalties did the trick for the Cardiff-based side, who are also hot on the heels of promotion in one of their best ever seasons. They currently lie seven points behind St Joseph's with four games left to play in the league.
They will meet Llantwit Major in the final, who had the big not of Ryan Evans to thank for their 18-15 win over Old Illtydians at Nelson RFC. The Illts dominated large parts of the game and had two tries from Phil Morse and five points from the boot of Adam Stanford.
But their discipline let them down and allowed Evans to punish them. Llantwit failed to reach the Illts 22, but kept their cool to reach their first final.
Llantwit are due to host Clwb Rygbi at home on Wednesday, 16 April in the league. They lost 26-18 io them away from home earlier in the season.