The home side started brightly, with Lee Wallace running at the Forfar defence before seeing his shot blocked.
However, the opening goal came just minutes later. This time Wallace curled in a cross and Nicky Clark powered his header into the net.
It was nearly 2-0 inside the first 10 minutes, but Clark's effort was saved after Lewis MacLeod had slipped him in.
Rangers missed a number of chances to double their lead, before Gavin Swankie fired into the top corner to bring the visitors level.
However, the Gers raced up the other end and Clark's shot deflected in off Darren Dods.
It was 4-1 just before the break. Ian Black hit the bar with a curling effort, but Clark pounced on the rebound for his hat-trick.