The Loons, who finished fourth in Division Two, made light of their First Division opponents and put them to the sword in the first half.
The game sprung into life just after the half-hour as Forfar blitzed Dunfermline, scoring all three goals in the space of 15 minutes.
Willie Robertson opened the floodgates with a rising strike from the edge of the area, before Christopher Templeman headed home Gavin Malin's cross four minutes later.
Templeman then struck a second on the stroke of half-time, again with his head, as he converted Gavin Swankie's centre.
The striker almost bagged his hat-trick midway through the second period - but Michal Hrivnak tipped over his drilled effort from the edge of the area - before Robertson blazed over from inside the six-yard box.
Stephen Husband netted a consolation goal six minutes from time with a 25-yard free-kick, but Dunfermline still have it all to do in the second leg.