GRID Autosport

This worked for me and many others.
Takes less than 10 minutes to complete.

Street Racing - Supercar - Race.
AI - Very Easy.
Pagani Huayra

Set all of the event tracks to Barcelona - Fountain loop.
1 lap is fine, the Pagani is fast enough to be able to overtake everyone in one lap.
If you're a beginner, set it to 2 laps, which should definitely be enough time to overtake!

Don't forget to install all vehicle upgrades in the lobby. Vehicle tuning isn't necessary.

This is a tricky acheivement as the K&N team rarely pops up.
Check each team offer (for every discipline) after every season as it appears to cycle.
Initially, For me, K&N appeared as an offer in Endurance for the 51st season.

K&N Motorsport also appeared for me as a team offer on the GRID Legends Series.
You'll unlock this series a long time before season 51, so it's possible to complete the achievement sooner.

This achievement unlocks when you level up a single car to Level 99.

Safest bet is to pick a common class and car. In my case, I chose a Golf R from the Hot Hatch category.

For reference, I probably completed ~150 races to level up the car.

This is a pretty easy achievement if you have another player - or even if you don't!

Set up a Splitscreen custom event with the following settings

5 events

C2 Drift class

Hockenheimring Drift circuit (For all five events)

This can be completed within ten minutes from menu to menu.

Create a Custom Cup event with the following settings:

Open Wheel - Super Lightweight - Sprint - AI Very Easy
The only car available in this class is the Caparo T1
Change the difficulty settings so you have unlimited flashbacks.
It's preferable to do this with manual gear changes, but absolutely not necessary.

When at the lobby, add all available upgrades.
In Vehicle Tuning:
Brake Bias: +20
Differential: -20
Downforce: +20
Gear Ratios: -20
Suspension Front: +0
Suspension Rear: +20

Best trick is to be ready to flashback at any time. Collisions (no matter how little) will ruin your time.
On a Sprint, there may be other cars ahead of you. If so, overtake cleanly, and again if there are any collisions, flashback.
Try not to brake on turns and keep a small bit of throttle going even when turning.

Create a Custom Cup event with the following settings:

Open Wheel - Formula A - Sprint - AI Very Easy
The only car available in this class is the Dalara Indycar.
Change the difficulty settings so you have unlimited flashbacks.
It's preferable to do this with manual gear changes, but absolutely not necessary.

When at the lobby, add all available upgrades.
In Vehicle Tuning:
Brake Bias: +30
Differential: 0
Downforce: +50
Gear Ratios: -40
Suspension Front: +30
Suspension Rear: +20

Best trick is to be ready to flashback at any time. Collisions (no matter how little) will ruin your time.
On a Sprint, there may be other cars ahead of you. If so, overtake cleanly, and again if there are any collisions, flashback.
Try not to brake on turns and keep a small bit of throttle going even when turning.

Mostly this is doable with just weekly events (took me about 1.5 years with that)
Other option is find a boosting buddy and do 1 lap races, whoever has the "driver to watch" should win as you get more xp then.
All assists off, AI as hardest level as you can handle as long as you win.

When doing the weeklys also do them step by step (First Bronze - Collect XP , then Silver - Collect XP, then Gold - Collect XP, then platinum - Collect XP) if you instantly do platinum then you miss out the XP from Bronze, Silver, Gold.