I did it with the first try with the following strategy:
- it shouldn't matter which GT500 car you use
- you don't even need to tune. maybe set the top speed a little higher because of the long straights providing slipstreams.
- start on intermediates
- with a high level bob, you should be able to run him between 75-100% of the emotion bar
- use 'overtake' when he gets close to an opponent.
- the wetness-indicator will go down to 20%, then up above 40% and then come down again to something like 22%. So it doesn't really matter which tyres you use after pitting (intermediates, racing hard, or sports soft as someone suggested, although that feels like cheating to me)
- my bob's pit-light went on just before the end of lap 7. I pitted, but it's better to give him the "don't pit"-command and go another lap. Because of all the pit-confusion and cold tyres, the AI will lose at least 15 seconds on the out-lap, enabling you to pit comfortably after lap 8, not lose any time in there, and come out far in the lead, even if you weren't the lap before.
Result:
Coming in, I was less than a second in the lead. I went on intermediates also after pitting, and the result was I came out 9 seconds ahead. I was in the lead 19 seconds after lap 9 and the gap wasn't even registered anymore at the finish, because the AI was too far back. As it is, the AI's emotion bars also go down when they run on their own, so they instantly start getting slower. Which is what happens in the laps after pitting, because the field is much more spread out.