You can race through level 2-1 and do the objectives without having to actually fight anything. Don't forget to hold that sprint button. Doing this on the hardest setting will net you roughly 30k xp every two to two and half minutes depending on your run and load times. You can just click Replay Mission at the post mission screen and have your xp and scrap still saved. This means you can take a chassis from lvl 1 to lvl 30 in under an hour.