Brackley Town manager Jon Brady said his side must learn from their loss to Colwyn Bay and use it as a catalyst for the play-offs.
Saints were out-of-sorts against a tenacious and hard-working Colwyn, who were well worth the victory.
Rob Hopley opened the scoring with a superb header before Jamie Ellison doubled the lead through a 25-yard free-kick that flew into the top corner.
A Tom Smyth own goal from Tom Winters’ cross gave Saints hope but Ellison struck again 10 minutes into the second half to put the game beyond Brackley.
Brady said afterwards: “Tonight, I thought we didn’t start right. We got punished with the early goal and from then on it was an uphill battle.
“I don’t usually like to say this but it was an uphill battle playing against 12-men with the referee and we all saw it tonight. To not even give a booking for all that time-wasting, even early on, was very weak.
“But that’s no excuse for how we played tonight. We know from tonight that if we start like that we will get punished against any team and that’s a sharp learning curve.
“Tonight is disappointing but it can also be the catalyst where we can turn round and say that won’t happen again.”
This weekend, Saints host mid-table Worcester in their final home game of the regular season.
Brady added: “If the players want to play in the play-offs they have to fight and want to show it.
“Worcester have been on a really poor run and I expect a reaction from us and three points.”