ill add a few tips in here.
for perfection, the more power plays you do, the more aggressive they will become. if the race is 3 laps, just race for the first 2 and on the 3rd, thats when you bring out the big guns and route change or whatever. then when youre in first, always have at least 1 power play ready in case they get you with something.
for heavyweight, i used the canyon. the first power plays with the grates can take out almost the whole field, do little power plays throughout the first lap, but when you get to the dam, you must have red to route change, then just hang back and keep taking out enemies with power plays. the most crashes i ever got on 1 race was 38 but my average was around 25. obviously you do this after youve got the perfection trophy since you dont want to win the race.
for convoy, im pretty sure you actually cant do this trophy in career mode because passing 12 trucks in a row makes your score way higher than the 1st place computer guy. the game immediately stops once you become the high score. so you need to go into quick play and play it in there since there is no high score but your own and will let you play for as long as you want.
for the online form of 75, you dont really want to focus on power plays. focus on being a fast racer since there is no rubberbanding AI, once youre ahead by a good amount, there is nothing they can do to catch up. i used the ryback firestorm.