You need to manipulate the standings to guarantee victory, though a little luck usually helps too. You can most certainly win the GP with 18 points as shown here.

What you want to do is, during every skip race, you want to manipulate the winning CPU into losing the race by causing that CPU to fall behind before you skip the race. The current race standings are used when you skip the race.

It is apparently not mathematically possible to triple-skip the GP on a 5-race GP.