Forza Motorsport 6

Go to rivals and press Y to see the leaderboards on a track you like (Ovals like Daytona or Indianapolis are good for this).

In the leaderboards scroll down to the bottom and choose the last one as your rival.

Before you start the race, go to your settings. ABS off, no rewinds, manual with clutch etc. (you can leave the steering at normal, if you don't like sim steering).

Beware: Do not have a mod loaded when entering a race!

The clutch is LB (at least with my settings).
When the race starts, accellerate and press LB and B (shift up).
Off you go, Remember to press LB everytime you want to shift (otherwise you will get damage).

Let's say, your rival drove a 2:30 (dirty lap, the ! indicates that). You should easily get a better time. Try driving a 2:27 or so (dirty! - hit a wall softly, if you have to).
You beat your rival and the next rival with a 2:25 (just an example) is set. So drive another lap, this time a 2:22.
If you can't drive any faster, drive a clean lap (without an ! in front of your time) and beat the rival with the dirty lap.

Repeat on another track (maybe the second oval track?).

I did that and got 5 needed wins with 5 laps on one oval track.

Super easy... Get a Tesla Model S, go race on one of the Top Gear short tracks. The Model S is an electric car, only has one forward gear (no shifting 😀). The Top Gear short tracks are super short and simple. Snap achievements if you want to track progress as you go. No need to select the rival at the bottom, unless the game has preselected one of your friends that's higher up the leaderboards. Make sure you start with a rival that has a dirty lap and set slightly better times each lap, to try and beat one rival per lap.

very simple. best solution right here. not sure why the others are so complicated, yet also leave out key info.

from the Go Race tab, choose Rivals, then choose Time Attack for whatever Class you like. go right to the Daytona Tri-Oval or Indianapolis Oval and press Y to show the leaderboards, then press X until you see Friends scores. pick one that has a flagged lap (triangle with ! inside). if you don't have any flagged laps, go back and choose Tri-Oval at night, or try another Class. Indy is shorter, but Daytona has softer curves which you may prefer since you won't be using the line.

now choose your car. if you absolutely cannot drive Manual, you can buy the Tesla car, which has only one gear so no shifting is required. however, all you need to do is go around the track once without messing up so Manual shouldn't be a big issue. at the pre-race screen go to Assists. you need the settings to be Off, ABS Off, Normal OR Simulation, Off, Manual w/Clutch, Simulation, Off.

all you need to do is complete the race with no collisions. if your time is not flagged then you will always beat the flagged time. if you mess up, just go around another lap until it says you beat your rival. now pause and choose Finish Race. go back to the leaderboard and select another rival with a flagged time, even the same one works. repeat until you reach 10.

The trick to this one is pick a track where you don't have any friends as rivals on the track. That means you don't have to manually select a rival and it saves time. I was lucky and none of my friends had driven on Daytona Tri-Oval at night.

Update: In the comments someone mentioned the default rival was the fastest player instead of the slowest rival I experienced. Be sure your default rival isn't the fastest, otherwise you have to run an extremely slow lap first to set an easier rival.

Like the others I used a Tesla Model S to avoid shifting, however I did a simple trick to do this in 10 laps.

My first lap I ran extremely slow but made sure I stayed ahead of the avatar and watched my average speed closely. For example, I ran Daytona at about 40 miles an hour. It's painfully slow but then the next lap I ran at 40, then at 50 and so on. The Tesla can average about 140 average speed with the right tune so this was pretty easy to do.

I've made a video to show how do this in around 10 minutes:

If you have played normally until now, most of the tracks probably have a "good" time set for you, or at least, one you might have trouble beating.

Here is the fastest method to get the easiest possible rivals.

Go to the rivals menu, then find a track/class combination you haven't done before. Check the bottom line on the right side of the screen, below the "rival recommandation" time : if it's empty, you haven't raced this yet.

I'd recommend a slower class, as the cars are much easier to handle : remember you won't have TCS, ABS... Low-power cars, FWD and AWD cars are much easier to drive without TCS, too.
Short tracks are better : having to do a long lap when you've missed the time by a few milliseconds is annoying. The shortest track is Long Beach West (0.72km), but the Top Gear variants and Catalunya School are pretty short, too.

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The first step is to get as low in the rankings as possible, to help you pick a slow rival in the next races. Scrolling to the "dirty" tracks from the top is incredibly long.

Start the race with a random rival, we don't care about it for now.
After the start of the race, cross the start line (otherwise the lap time recording won't start), stop the car, wait for a while, a few seconds, or even a few minutes. Make yourself a sandwich. Go get your mail. Walk the dog.
Once you're bored, run the track in first gear, slowly.
Make sure you (gently) hit a wall or go in the grass, to make your lap "dirty".

Once your lap is complete, finish the race to save the time, and go back to the rivals menu.
Amazingly, even though you waited doing nothing, your lap will probably not be the slowest one

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The next step is to very slowly increase your speed to easily beat the rivals.

Open the rival selection in the leaderboards (), select "near me" ( twice), pick the time just above yours, set as rival, then run again.

Waiting a while in front of the finish line to beat the rival by "just a few seconds" can get boring quickly, so you can "just" run veeeeeeery slowly this time. Don't even shift to second gear.
Next time you'll just run a liiiiitle bit faster.
Also, remember to hit a wall or go in the grass to keep your lap dirty !

As soon as you beat the rival, finish the race, go back to the rivals menu, select the next one, and make another very slow lap. If you let the game pick a "next rival" for you, it won't be the one just above your time.

You should be able to beat 10 rivals easily, by slowly increasing the pace each time.

If you set a "good" lap time by mistake, and you have trouble beating the next rival, just find another track/class combination you haven't done before, and repeat the operation.

I may have found a fast and simple way to get the unrivaled achievement. First step is to pick your rival preferably a friend and I did this on the NASCAR Homestead oval. Once I set my rival I went into the game with to the assist and turned everything off except for the steering. Also remember that you are using manual with clutch so remember to use LB and B at the same time while accelerating. Once your assist are set start your race and beat your rival and once you beat your rival finish the race but here's the thing don't quit just restart. Rinse and repeat 10 times until the achievement pops.

Here are my settings from my assists.
Suggested line off
Breaking ABS off
Steering normal
Traction control and stability control off
Shifting manual with clutch
Damaged Fuel and tire wear simulation
Rewind off

Hope this helps and good gaming.

ACTUAL EASY METHOD

1. Select the Telsa Model S

2. Go to one of the Top Gear tracks

3. Turn off all assist, keep steering on normal

4. Drive ridiculously slow. Finish the lap, quit out and then go back to the menu.

5. This new slow time is now your rival. Just drive to the finish line and wait until it gets near the timer and then drive over. Your next lap will also be very generous. Repeat 9 more laps.

It's really that simple. Why scroll through endless leaderboards, worry about dirty laps and other over complicated trickery when you can just set yourself as a rival and control the times you are up against.

Similar to above suggestions turn off everything man w/clutch etc. I chose daytona picked a random rival class to race in (this instance B Class) drove at race pace brushing the wall after the back straight timing marker ensuring next lap would be dirty, then stopping just before the line, then crossing the line knocking a second off each time LAP 1.22 cross at 1.21 n so on till get to lap 9 then pull a couple of clean laps to put yourself on the board. good luck & hope it helps