Armstrong and Greg McKegg scored in regulation for the Leafs, while Bill Guerin and Marc-Andre Bourdon replied for the Flyers.
Leafs netminder James Reimer and Flyers goalie Sergei Bobrovsky were both terrific in a shootout, a lengthy affair that went 11 rounds.
Armstrong, a free-agent signing in the summer, tied the game at two in the third with a power-play goal, re-directing a Danny Richmond wrist shot past Bobrovsky, who began the third period in place of Brian Boucher.
Kris Versteeg registered his first point as a Leaf with an assist on Armstrong’s goal.
Versteeg and Armstrong both had stronger efforts in their second appearances of the pre-season, but their centre, Nazem Kadri, was taken off the line after another sub-par effort.
Kadri did impress his hometown fans with a shootout goal after a nifty deke by Bobrovsky.
The former first-round draft pick had the highest expectations of any prospect coming into camp, but has struggled to raise his game to the NHL level, and will need to make more of an impact offensively if he is to remain with the Leafs when the regular season starts.
Reimer made his case to be the starting goalie of the AHL’s Marlies this year with an outstanding third period and overtime in relief of Jonas Gustavsson, his biggest save coming on a James van Riemsdyk penalty shot.
McKegg, his first game in a Leafs uniform, evened the score at one in the second period. The 2010 draft pick from nearby St. Thomas, Ont., scored just seconds after Boucher made a terrific save following a Leaf three-on-one opportunity.
Guerin, in Flyers’ camp on a tryout, gave Philadelphia a 2-1 lead moments after McKegg’s goal, a fortunate break when his centering pass went off two Leaf defenders and past Reimer.
The Flyers opened the scoring six minutes into the game when a shot from the blue-line by Marc-Andre Bourdon went off the endboard, hit Gustavsson’s skate and in.
Gustavsson bailed out the Leaf defence late in the first period when Scott Hartnell broke in on a break away, but the netminder made an excellent save with the right pad.
Brett Lebda left the game in the first period, forcing the Leafs to go with five defenceman.
Richmond benefited from the extra ice time, helping the Leafs forecheck several times by pinching down the boards.