This can be achieved in any race at any difficultyAfter selecting your car for an event the game will load the service area from here select
Difficulty and Assists then Advanced Settings.
You will need to turn Damage
OnOnce the race starts look for a section of the track with a wall or some trees to crash into, you will need a bit of speed to achieve the terminal damage but you will know when you have since there will be a cut scene.
Note you can also drive into deep water and recieve Terminal Damage Credit to OCChoppers for pointing that outSelect Flashback from the new menu on screen, rewind to a safe point and press
to use a flashback, now carry on the race as normal,
You must win, so i suggest having the difficulty set to 1 to make this easier.
Here's a video to demonstrate me wrecking a car