Hotshot Racing

I was able to get this on the second grand prix - Pro Circuit using the Thunder.
Winning every race in the grand prix isn't needed but you will need to win overall, so I recommend restarting the first race if you don't win to give yourself a better chance if you don't do very well in the later races.

A few tips:
-Start holding RT as soon as you see the 1 on the countdown, this will give you a boost launch every time
-Be sure to make use of slipstreaming (driving behind another vehicle) to increase your speed and build boost
-You can 'latch on' to other vehicles and be able to go as fast as they're going by driving into the side of them or getting pushed from behind, but this may cause you to crash or even spin if you're unlucky
-Practise the tracks in time trial or on a lower difficulty if you are struggling
-Have a bar of boost left over at the end of the race in case you need the extra boost to get the win
-The AI can seem very broken with rubberbanding near the beginning of a race but you will most likely catch up to them near the end

This isn't my video but it shows the general routes through each track, full credit to Marius Urucu for the video

I got this one on GP1 with the Mirage. The important thing to keep in mind is that you don't have to be perfect - this game isn't as brutal as other arcade racers where the same opponent gets second every time you get first. I unlocked this achievement by taking first, first, third, and then fourth, and I still took 1st by 4 points. I raced and restarted the first map until I got 1st, then just hoped for the best on the rest of the GP.

For the most part, this strategy works with any driver's car that is strongest in top speed. Focus on being smooth, not on trying to go fast. If you're smooth, fast happens almost by accident. You don't need to be perfect. I hit my fair share of walls.

For a solid launch, keep the RPM's in the yellow between 6 and 7. A good launch should put you into the top 5 - 1st if the other racers can get the hell out of your way. From there, it's mostly don't get left behind. So long as you can reasonably see the leaders, you're not out until it's over. I've fallen back to 6th and still taken first several times.

The arrows along the side of the track are your friend. If it's fixed arrow signs, you can almost always get through those without brakes or letting off the gas. A reasonable attempt at a racing line is all takes: start turns from the outside, cut inside through the apex, end the turn back on the outside, etc. If you don't know what any of that means, fire up Forza and follow their guide line for a race or two, you'll see. Illuminated/animated arrows along the sides means you can expect to drift. U-turn arrows, you can bet you're drifting.

When drifting, try not to oversteer. You'll maintain the most speed if you can keep the car pointed so that at the end of your drift, you could almost take your thumb off the stick and the car is already lined up exactly where you want to go with minimal corrections. The sharper turns will require a hard cut in, then back out, but on most of the drifts you can line up your exit pretty easily. Don't force a drift, if you can take a turn by not drifting, don't drift, but it is more "expensive" to hit the wall than it is to drift, so if a drift gets you out of the hit, drift.

Slipstreaming is pretty key to keep with the pack, as it gives you a crazy extra amount of speed, as well as helps charge your boost. Since you'll want to minimize your drifting, you'll want to be sure you're slipstreaming as much as you can so you have boost when you need.

If you hit a wall, a boost can get you back up to speed pretty quickly. If the race is looking tight, you'll of course want to sit on one for that last stretch.

Again, you don't have be perfect, don't get upset and quit just because you took 4th in the third race, a solid finish in the final can still put you at the top of the podium.