Instructions
Play through the Pacific Northwest section of Cryptids Island until Gretchen captures the Bigfoot from a cave. Fly your helicopter close to her helicopter to intercept it, and the perspective will change to show you on the railing of your helicopter.
Drop down to the left off of the railing, and you'll land on the cage holding the Bigfoot. Jump from here onto the railing of Gretchen's helicopter. Click the gas tank near the front of her helicopter to drain it of fuel.
Drop back down to the top of the Bigfoot's cage. Go into your inventory and click the gardening shears to cut through the rope connecting the cage to the helicopter. A parachute will open, and you'll both drop down to the ground unharmed.