For short sections like the ones you noted, sometimes you just have to stand up and gut it out. I could tell that is what you did in the video so really that's what it takes. Sometimes I'll just stay seated and lean over the bars in a low position and try to stay as smooth in the pedal circle as possible. Either way 13% is gonna hurt. The more often you do really steep sections the better you'll get at them. As a side benefit, the 6% stuff will seem a lot easier too.