Mountain pulled off a resounding win to progress to the Ardal Northern League Cup quarter-finals.
Goals from Sam Jones, Ben Brierly, Gary Roberts, Kyle Smyth (2) and Sam Molyneux made sure the Flint side keep on winning.
At the other end, Jamie King saved a penalty to keep a clean sheet.
Builth fought back from a goal down to overcome visitors Penycae with relative comfort.
Danny Holland handed the away side a 30th minute lead, but by the interval Lyndon Jones, from a free kick, and a Lee James header had put the Bulls in front.
Another Jones free kick on 53 made it 3-1 and things got worse for Penycae when they had Evan Woodall red-carded after 67.
The hosts wrapped the tie up when Jamie Evans added number four on 78 minutes. Liam Sklenar bagged a late consolation for the away side.
Goals scored by Craig Harris (2), Norton Collins (2) and Luke Evans ensured Caersws cruised into the last eight.
A good day for the Bluebirds was enhanced by 16-year-old local lad Sam Lewis making his first team debut.
Llanfair earned themselves a quarter-final trip to Denbigh Town with a welcome cup success at Llanuwchllyn.
Karl Seliaerts, Nyasha Mwamuka and Jack Jones pocketed the all-important away goals for the Powys visitors, Mike Pritchard (pen) and Tom Jones replying.
In a tight game with not many chances two late Sion Ewart goals were enough to ensure the Magpies advanced to the next round.
This tie was reported on in depth by Grassroots North Wales on Friday evening.
Captain Marvel Paul Fleming picked up a hat-trick for victorious Denbigh, with Corrig McGonigle’s 35th goal of the season not enough to save 1876.
Bow Street v Flint Mountain
Caersws v Builth Wells
Denbigh Town v Llanfair United
Y Felinheli or Llanrwst United v Llangefni Town or Saltney Town
Dates for ties to be confirmed
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