I had the same issue. I got to Step 14 with no problems and then stripped the bottom left screw head! I was able to get it out with the help of a rubber band. You lay a section of the rubber band across the stripped screw, place your screwdriver on the screw pinching the rubber band. Then place a decent amount of force to engage the screw as you turn. Worked like a charm for me! Also, if you bought the ifixit optical bay hard drive enclosure they included two of those screws in the bag of screws (at least they did in mine).