This guide will apply to version 1.0.0.5, the current version as of this solutions writing. If you found something in here that has changed or is incorrect, drop me a comment and I'll be quick to correct it.
This achievement will require a little skill, a lot more luck, and a small mountain of patience. Even then, I think I've found a method to minimize the necessary skill, leaving that bit of luck and patience to you.
First bit of advice: If you're intent on completing 8 to Glory, do this achievement as part of the slog to complete 100 rides in Arcade Mode for the "Practice Makes Perfect" achievement. I'd also recommend that you complete the campaign and grind out a full set of +100 gear (the end of campaign advice given by SH4D0WWULF in his "Maxed Out" guide is a good way to get extra packs) before attempting.
I used a level 5 J.B. Mauney as my rider as he has the highest stats, with each of the three +100 Champion gear cards equipped.
The three scores at the end of the round determine your overall score. Composure seems to measure your button pressing skills (the forward balance and the Bull combo), Balance measures your ability to stay upright through the bull's turns (the

control), and Strength (grip) determines how detrimental a missed button press or balance cue is to your rider. In short, you'll need to be at or above 95% across each of these scores to hit the necessary points at the end of the ride. The number you're trying to hit is 45.75 Rider Score, and 44.25 Bull Score. The game's difficulty setting seems to have NO EFFECT to your overall score. It does not include a multiplier for engaging Magic Train at the Hard or Pro levels. Do this one at Easy.
Grip: Starting the ride
You're aiming for perfect on each of the three sections. With the Easy difficulty setting, the slider moves slower, giving you a better chance of hitting.
Opening the gate:
Here's the opportunity to take a little skill out of the ride - just hold

left for the full ride (don't need it during the combo, but hold it left once you're done with the combo for the final two forward balance cues). Consistently, I find that my Rider vs Bull bar stays mostly blue for most of the ride. This is the Balance stat at work, and I've found that simply by holding LS over, I can often (not always, there's still some luck at play) get 96% on the balance stat at the end. This trick doesn't work for every bull, but does work for Magic Train. The right side of your screen will flare red, and you'll get that helpful hint to tap LS to the right, but ignore it. Hold left the whole time.
Make sure also to be consistent with your forward balance cue - this is the moving red circle under the bull. You need to hit each of these rings accurately to push your composure rating to the highest at the end of the ride.
The combo:
Each bull's combo is unique and unchanging. Memorize Magic Train's combo, and focus on getting either 'Great' or 'Perfect' in that sequence for each button press. The only thing that changes about Magic Train's combo is the speed at which it is thrown at you. The overall accuracy of this combo and the forward balance factors into your composure rating. Magic Train's combo is:

If you memorize this combo, you can concentrate on making sure that you're hitting it with near perfect accuracy, rather than reacting to the falling buttons.
The end:
Your rider score needs to be at 45.75, and the bull score needs to be at 44.25 for a total score of 90.00 (I've never accomplished more than 90.0). I've only ever seen two values for the bull score, 43.25 and 44.25, and frankly I don't know what drives it. I've seen great rides with a bull score of 43.25 (resulting in less than 90.00) and terrible rides with 44.25 (also resulting in less than 90.00). This might be luck based.
Have a suggestion on how I can improve this guide? Leave me a note below!