Wreckfest

You can idle for 3x20mins in a 0 AI deathmatch but why do this the boring way?

I recommend grinding this after you complete the career. Chances are, you still don't have these unlocked at this point:




Great then! Pick Lawnmower (first upgrade its engine to have some advantage over AI), Fairfield County track, deathmatch, 20 mins (or how much time you want really), 23 AI, set AI to expert, switch off assists and... just have some fun wrecking these garden contraptions and don't be afraid to wreck yourself. All that counts towards the aforementioned achievements. You should get tons of XP at the end of the match. I played 10mins and got over 30k XP, 6mins and got like 25k.

3x20mins should grant you the "Garden Variety" and provide significant boost to the three others. Even if you don't unlock them all at once, it's still better than doing everything separately from the beginning to the end.

For this achievement, you will need to use the Lawn Mower (That you unlock in the very first race in career mode) for 1 hour. All you have to do is to go into a custom event, use the Lawn Mower, Then make sure the Ai is off, Set it to a race, Then just go afk for 1 hour and come back. Then you should have the achievement!