You need to reach 100% in each of the five categories to obtain this achievement.
At the end of each daily test, you will receive a grade in each category based on your performance, and your brain usage will go up or down in each category based on this grade. You will need to reach at least the middle of the C range to go up a percentage point. If you reach the B range or above you can increase by 2 percentage points or more. If you fall in the F range, you will go down a percentage point in that category. The final brain usage is the average of all the categories.
Once you get closer to 100%, there will most likely be categories that you are maxed out at 100% while 1 or 2 categories are lagging behind. At this point your total brain usage will go up much slower since some categories are maxed out, and the only way to increase it is to increase your percentage in the ones that don't have 100%.
You can view your brain usage in each category at the end of each daily test by scrolling to the bar graph labeled all stats. You can also view this in the brain charts section.
Once the challenges get harder, I would focus on getting the question right vs doing them fast. Getting a few questions wrong hurts you a lot more than doing them a bit slower. You can also exit out of the daily test if you know you are doing poorly so that your percentages don't go down. Just hit start and exit game and your stats won't be affected.
Other than that its just a matter of getting familiar with the games and developing a strategy for them. Everyone has their own strengths and weaknesses, so I'm not going to get into strategies for particular games, but personally I had the most difficulty with the first category.
Edit - Suggestion by Dancrozier:
If your struggling then some games can be made easier with use of the start button. This will pause the game and give you more thinking time. Good for the maths and calculation games, not so good for the visual ones. When they get really tough at the end then a good old fashioned pen and paper can help if you pause, write things down etc.