Load Stadium Races. Click Anaheim Stadium, choose a truck (I used the first one).
Just follow the green line. I used LT as a handbrake. Scored 11:679 and got platinum.
Load up Anaheim Stadium 2. Same as the first race just slightly harder. I used Mutt but it doesn't seem like there are any differences between the trucks except for looks.
I scored 12:846 and got gold.
Load up Sam Boyd Stadium. Another easy one. I scored 14:797 and got gold. Even rolled my truck once and still won.
Load Sam Boyd Stadium 2. Try and go a little slower off the starting line. Starting is the hardest part. I just lightly went over the first jump and was fine after that. Just watch out for your opponent coming the other way. 13:701 was my gold time. Took a few tries, just master that first jump and you're good to go.
Load Everbank Field. Very easy track even with messing up turns. I scored 16:335 for gold. Take the turns slow and use LT and you should be able to get under 13 seconds no problem.