Pumped BMX +

Firstly I suggest you always try to make sure you complete every run you start, even if you don't accomplish any challenges to minimise the grind later on. Once I completed all other achievements I was on around 350 runs completed, without using the aforementioned tip. Despite this you will probably still need to grind a couple hundred at least so I recommend loading up the first level of Local, 'Starter', and blitzing through this as fast as possible. You can complete a run in under 10 seconds and restart with little to no delay. It's a good idea to finish off any Tables or Barspins you need to do while doing this. You can get around 5-6 tricks done per run.