You need to build the fan that opens the doors in the starting room of transit. You need the manican body, plane rudder, and fan. The locations are shown in this video. Then you need to buy Quick Revive (500 Points) and open the two doors to get to the bus with the fan. Do it with the fan because it won't cost. Then ride the bus to the farm (Second Stop) and open the gate. Once you have opened the gate you can run into the barn and use the fan to turn on Double Tap and buy it (2000 Points). You should be able to do with by round 2 if you maximize your points.
Happy Hour Achievement Guide - Youtube Walkway