I got this while going for the advanced degree achievement.
I wasn't even trying to get the platinum achievement, but I got it anyway, so I would recommend going for that achievement first and see if you get platinum on the Adrenaline Degree tournament.
I just rode the Adrenaline Degree Tournament once and I got both achievements. I used the 450cc bike as I find it easier to control than the phoenix bike and it is easier to drive without mistakes on everything medium or less.
I know it sounds more difficult to pull it off in a tournament, but there are only 2 tracks and to be honest, I found it quite easy. It that was my first platinum in trials evolution, so I'm not that good either. I tried the separate tracks to get gold on them, but there I never even got close to platinum.
If you have trouble with getting a platinum medal on the separate tracks, I would recommend trying it in the tournament. I don't think you have to get platinum times on both tracks, but the platinum time on the tournament is set easier then the platinum time on the saparate tracks.