The solution of PJTierney is good but you can also try another circuit see below.
Head to Grand Prix and select the following:
*Track: Le Castellet (French Paul Ricard Grand Prix) This circuit is much easier then monza if you cutting corners.
*Race Length: 25% (13 laps)
*Rules and Flags: Off
*AI Difficulty: 101 to 110 (Ultimate)
*Assists: free choice (i don't use any)
Use the team you like (i used Williams)
My Fastest Laptime 1:14.926 Fastest Ultimate AI Laptime 1:32.660
With these laptimes you can do 4 pitstops for the improvise-adapt-overcome-achievementThx xxxxPRODIGYxxxx for the tip below
If you choose Renault you can also unlock the homecoming achievement.
The Homecoming achievement in F1 2018 worth 29 pointsFinish in the points at Le Castellet in a Renault