Prolific outside half Gareth Roberts opened the scoring with an early penalty before centre Liam Phillips crashed over for the game's only try. Roberts converted and added further two penalties to hand his side their best win of the season.
Ystrad Rhonnda are now breathing down the leaders' necks after a 26-5 triumph at Pill while Abercynon also remained in the chase after they edged a thrilling contest with Newport HSOB 33-30.
Elsewhere, Rhydyfelin recorded an impressive 31-6 bonus point win over Mountain Ash, Tredegar scored two tries to beat Builth Wells 12-6 and Llanishen were smashed 43-15 by Ynysybwl.
Abergavenny retained top spot in Division Three East as they beat Croesyceiliog 22-14 with Stuart Davies, Will Johnson and James Williams all getting tries while No.8 Dorian Green starred for Garndiffaith with two tries in a 32-0 drubbing of New Tredegar that keeps them second.
Brynmawr also remain in touch after Gavin Knapp and Sitoh Nobitano collected two tries apiece in a 34-0 triumph over Pontllanfraith.
Only two games survived the weather in Division Four East which allowed leaders Gwernyfed to extend their lead at the top over second placed Abertillery with a 27-0 win at Machen. Nantyglo moved level with Abertillery in second after a 21-13 victory over Tredegar Ironsides.