Super Monkey Ball: Banana Blitz HD

This is an incredibly boring and grindy achievement. The Decathlon puts you through all 10 minigames in a row. Any difficulty based minigame is played on normal. I timed one average run through, and it took me 11:40. Multiply that by 36, and that's a total of 7 hours if you never stop or lose focus or get bored. I highly recommend listening to something or watching something during the boring parts where you just wait out timers.

Fortunately, it is possible to unlock every minigame achievement in the game during the decathlon despite the fact that none of your records are saved. So you could save some overall completion time by grabbing some of the minigame achievements at the same time as the decathlon achievement. Below are the list of minigames along with the achievement standard and my strategy to get through them as fast as possible. Minigames marked with * indicate that my strategy is valid to obtain the achievement. If you want more detailed strategies for the minigames without asterisks, please visit the respective minigame achievement guide.

Hurdle Race* (20 seconds) - spam LB/RB alternately as fast as possible, and don't bother jumping with A, because the speed loss for hitting a hurdle is negligible. You will most likely lose time trying to time jumps because it will slow down your LB/RB spamming. I regularly finished in 12-14 seconds without jumping at all.
Monkey Snowboard* (1:40) - Get an initial speed boost by holding down on the left stick during the countdown, then hitting up the moment after the race starts. Hit as many bananas as possible and jump on every red ramp for speed boosts, but avoid crashing at all costs, as that is a huge time loss. I regularly finished in 1:30-1:45. The achievement time is not too hard if you complete a run without crashing.
Hammer Throw (110m) - Spam A while rotating the left control stick.
Monkey Target (2500 points) - Hit A as soon as you leave the cannon, then hit A again as soon as you lose all upward momentum. The meter on the right shows you when you stop moving up. Don't spam A because you can only open and close the ball once.
Slingshot (2500 points) - Wait out the 1:00 timer.
Space Monkey Attack (5000 points) - Purposefully die 3 times by running into enemies or bullets.
Hovercraft Race* (1:30) - TAKE IT SLOW. You will save a ton of time by moving slowly and methodically, as the vehicle is purposefully extremely difficult to control and turn. The route I took was to immediately turn left until you are aimed at a flag, hit that flag then continue on and hit all flags on the outer circle, then turn to the center when 3 flags remain and grab the rest. I regularly finished in 1:00 - 1:15. The achievement time is fairly lenient.
Seesaw Ball (8000 points) - Hold A to make the screen scroll to the top faster. Hold left once the game starts, then right once you hit the first black and white seesaw. You should finish in ~10 seconds.
Dangerous Route (2:00 left on clock) - Wait out the 1:00 timer. Collecting bananas/falling out/trying to complete the route will all cause the game to take significantly longer than 1:00.
Whack-a-Mole (3000 points) - Wait out the 1:00 timer.