Sonic & All-Stars Racing Transformed

Simply end up being on the first place podium at the end of the Grand Prix tournament and you will unlock this achievement. It can be unlocked on any cup, on any difficulty so do ut on easy if your having trouble unlocking it.

GENERAL TIPS:

For many people going for the 3 stars/ "A"/Hard or even 4 stars/"S"/Expert difficult (after unlocking it) it's going to be impossible to win at first. That's because the inicial vehicles mods available are kind of weak, their bonuses aren't as good as you need in terms of speed, boost, etc. Also because on those difficulties you need to finish in 1st in order to get the star, unlike the medium one/2 stars, where you can finish up to 3rd to qualify.
So, have in mind that failing in this game is not a bad thing, it simply means that you're missing some knowledge you gotta look for (shortcuts, how to use drifting, boosts, etc), things you can and will improve on. The good news is that it doesn't take A LOT of dedication for many of them and also that I've written this piece about the things I've figured out so far, so you can have an easier time if you're feeling too frustrated with the game.

1) Play the whole campaign, go for 2 stars on each map
So, first of all, go for only the 2 stars/"B"/medium difficult on all maps. That way you'll unlock a lot more characters and mods for the vehicles you already own. Genesis mod, for Sonic and a few other characters, is a good example of improvement that will help increase your odds at winning on higher difficulties later on.

If you can't get 2 stars on special courses like Traffic Jam, Drifting or races against shadow enemies where you have to beat their best times (forgot the name), it's okay to lower them
to just 1 star in order to be able to just move on since those can be a big tougher.

By the way, the special courses where you battle a single Tank are not hard at all, just tricky. Try to attack it when you're on the same level and on a straight line preferably. The main thing you've gotta watch for is not to die after being hit too many times. You DO NOT have to keep up with the tank at all times, so if your health bar is low, fall back, keep collecting hearts for a while to get your health back up and then go for the tank again. Falling back is fine, just don't do it too much FOR TOO LONG, otherwise you automatically loose it.

2) Which character to select/what stats matter most
I've tried many characters but not all of them. If you're wondering if Sonic is the best character game, I don't think he is. Neither is Knuckles. Tails can be good, maybe a bit better than Sonic later on... The good thing about Sonic is that his vehicle feels stable and he can be fast as expected. But there are faster chars than him. Knuckles tricycle on the other hand is harder to control. I'm just pointing out these chars as examples.

You can be fine playing with Sonic while going for 2 stars on all maps like I did for a lot of them. Many of the characters can be just okay, until you feel that they're not good enough for what you specifically need at the moment.
Shadow might be one of the best characters though. One of his mods make him fast and with a good boost (which in my opinion are the 2 most important stats) and not too bad to control. From the initial 5 Grand Prix, I won the first 3 with Shadow and the 4th one with Amigo (mexican dude). I had to change to Amigo because even though my control of Shadow was fantastic at that point, the maps on the 4th GP had sharp curves that required a more maneuverable char. Amigo accelaration wasn't too good I think, but once I got to 1st place on a map I hardly hit any curbs/edges/walls on the sides, nor fell off the map because it was so easy to control him, so I kept my position with ease.

Acceleration and the bonus pick up stats aren't prioritary stats, in my opinion. Of course that to have a more balanced vehicle is ideal, so if you can have a char/vehicle with 4 or 5 points of speed, 2 or 3 of acceleration and maneuverability, 4 or 5 in boost and no matter how many on bonus pick ups that's seem to be a good one. The reason I think that bonus pick up isn't as necessary is because even when I played with chars that only had 1 point in that stat I was still able to get the All-Star bonus in almost all races, which is the best bonus.

3) Small boost at the beginning
At the start of the regular races keep pulling your right trigger at medium speed. When done right, you'll notice your exhaust pipe will change color and this will give you a small boost. On the other hand you don't do it, you'll notice that all the other competitors will gain a good initial advantage from you at the beginning of the race, because they did it and you did not. After the pipe change colors for the first time you have 2 options: a) Stop pressing the RT and then just holding it and letting it go once the race starts or b) Every time the counter goes does (3.. 2... 1...) you keep trying to get the pulling of RT right again. If you do it right it will change colors again and again. I think it goes pink first, then purple and then blue. This boost is nothing major and you only feel a bigger difference if you get to the 3rd level, the blue one. But like I said, the major reason to try to do this is not to fall behind too much at the beginning.

4) Learn how to drift
If you're like me, not a fan of racing games, then you might have a hard time on curves and letting go of gas and using breaks and all that. Well, you need to overcome those limitations to progress and it's not too hard at all. A great tricky that I've learned and that makes my races A LOT easier to win now is holding the Left Trigger to drift while turning right or left on a curve. If you hold it for long enough you'll see your pipe go red, which means you auto-generated a boost. Let go of the LT after you are done turning. When you learn to use this you'll understand why you always fell back comparing to the other AI racers even if you went through a curve without hitting the lane's edges. You'll start drifting a lot, almost everytime you can.

The second part of the trick is to actually start drifting just a bit before you actually start turning right or left on the curve. That way you'll be able to go through it with ease and will surpass/overtake the other racers a lot more often too.

5) Saving the "All-Star" bonus until the very end
Mark my words: this can make you win tough races even if you are behind during the final lap. Using the Star bonus before the end is a waste since for small mistakes like hitting curbs will make you fall back positions.
Of course it'd be ideal if you got the Star Bonus later on during the 3rd and final lap, but in case you get it on the 2nd or even on the 1st, consider saving it. The downside is that you won't be able to pick up and make use of other bonuses, but that's fine. Focus on the race as best as you can.

Let's say that the All-Star special effect lasts 10 seconds. So you know that you need to save that bonus for the last 10 second-part of a map. Doing it before might put you on the 1st position earlier, but then you risk being overtaken by some BS from the AI players. So, it's best to use it as explained above.

The good thing about the Star bonus is that it makes you move forward and on track almost by itself. Also, if you hit the A buttom every once in a while you can perform specials on the track that can take out enemies that are close to you or lay traps on the course which can help to make sure others behind won't catch up.

Well, those are the great tips I have to share at the moment, things I've learned so far after playing the game for a bit over 20 hours and that are ensuring that even S difficulty maps are winnable. I hope this helps you too. Consider leaving a thumbs up or comments to let me know. If you know any tips besides these ones, consider sharing them. Thanks.