Need for Speed Hot Pursuit Remastered

You can quite easily earn this achievement during the event called Comeback Tour which is a racer event in Coral Bay on the coast. The event puts you in the Diablo automatically. About four miles from the finish there is a very long straight where you can get the Diablo to top speed. The benefit of doing this event is it is available likely before you have enough bounty to unlock the Diablo.

When on the career map select the racer icon on the bottom left titled "Grand Ocean Coast" and press the "Y" button to start racer free drive. The select exotic and select the Lamborghini Diablo. The car will unlock through progression. If it is not unlocked it will tell you how much remaining bounty you need to unlock it. Then just drive straight from start and build up as much boost as you can around the few turns. Once on the straightaway use boost till you see top speed and you mph/kph is in the red. The achievement should pop after a few seconds.
Hope this helps.