Forza Motorsport 2

I never started 8th in career, so to get this I went into in an Arcade room on Xbox Live. I created a race using 7 AI opponents, put it on one lap, and for the starting grid order I set it to Lobby points so that the higher you finish in the race, the closer to the front you'll start. When you start the race, lose on purpose and come in 8th. In the next race, you'll start in the back. Since you have the lowest Lobby points, you'll start in 8th regardless of what kind of car you pick.

Alright, there's a lot of good advice given here, however they don't say what race to do.
I was able to knock out this and the "Underdog" achievement in one go. What you'll want to do is buy the VW Golf GTi Mk2 and race in the "Hot Hatch Runoff" playlist in the "Proving Grounds" tab. You'll want to select the first race there. You have to be a lvl.3 to get into that playlist, but it doesn't take long to get there. also you'll want to set the difficulty to the lowest. By the end of the first lap I was in 1st place.
Good Luck, I hope this helps.

If you enter the race MId Engine challenge with the 1970 Porsche you win for an earlier race then you start in 8th position. Basically on the very first race Inland Taipei reverse, on easy, you can get the achievement. The other cars are terrible on the tight linch pin curves on this track, so on the very first set you can pretty much bounce your way past the cars. Your car is slower, but right after the linch pins, you can basically gun it with the Porsche, even on the section with red basically through the large C curve, by that time you should have a big enough lead to hold on through the 3 races. I actually won with a margin of over 10 seconds on the next car.

Just make sure you make no adjustments to the Porsche because I dont know if that will affect your starting position. It almost seems the game is kind of stupid and puts the worst car in the worst position giving you a double handicap, on the flip side I have often been in first with best car.

You just have to use a car with the lowest rating possible, just check before you start a race the cars' ratings of your opponents and choose a car with a lower rating and you will start 8th.

I know it seems unfair (the worst car in last, the better in first) but it is like that.
Also set Easy Difficulty and some assists if you need it.

You can earn the Underdog achievement at the same time.