Forza Motorsport

You can select the worst lap time and do 20 laps with the same rival. It is not necessary to change rivals or circuits.

You can rent a car if you don't have one powerful enough

I did it in Class X on the Indiana oval This takes approximately 45 seconds per lap.

An other way is to take Class X Porsche on Eagle Rock Speedway is around 24 seconds a lap, takes under 8 mins for 20 laps. (Thx Wyyvern for this)

Not sure if this will get patched out but there is a very easy way to do this without having to scroll through any menus or driving deliberately slowly.

In the Race menu select Rivals. Choose whichever class you want and then the course and layout. Ensure it is a layout that you have not yet set a rivals time for.

You will be presented with 4 boxes, one with a select track option, one with your time which should be blank, a box with your rivals details in and then a box with the leaders time. If you highlight the box with your time you will get an option to set as rival, doing this will blank out the rivals box as well as you are now your own rival. You can then proceed to do the rivals event and will get credit as soon as you finish your first lap.

If you do a dirty lap on your first lap then you should easily beat your time on the 2nd lap due to having a rolling start. Ensure your second lap is also dirty and you should be able to beat that time on your third lap simply by doing a clean lap. Repeat until you can't get any faster and then do the same process on a different course.

There are ovals at Eaglerock Speedway, Homestead Miami and Indianapolis Motor Speedway. It should be easy to get at least 3 at each of these in next to no time meaning you are almost if not more than halfway to the achievement in less than half an hour.

Go Into race and then Rivals. Pick any track. In setting, change Global Assists Presets to Maximum, then change Throttle to Assisted. Start the race, let the car do most of the work and use the accelerator if you need a boost to beat a rival. Once you can't beat the rival easily, back out and choose a different track. Keep doing this until you earn the achievement.

You will also unlock the Enthusiastic Rival achievement


You will also unlock the Experienced Rival achievement


Here's a video Guide :

Go Into race and then Rivals. Pick any track. In setting, change Global Assists Presets to Maximum, then change Throttle to Assisted. Start the race, let the car do most of the work and use the accelerator if you need a boost to beat a rival. Once you can't beat the rival easily, back out and choose a different track. Keep doing this until you earn the achievement.

You will also unlock the Enthusiastic Rival achievement


You will also unlock the Experienced Rival achievement


Here's a video Guide :