Make Way

This is quite a grind, but to boost set up a local game with bots and put the bots difficulty on training. Once you unlock level 10, go to custom match to toggle more settings to help you (no hazards, walls on, etc.). This should be easy enough to hit every check point and win each time, allowing you to rack up the most xp in one match. You also get XP for hitting opponents with various weapons or traps, as well as building tracks, hazards, etc. Using this method, I was able to quickly hit level 25 in a few hours, but know that after 25, the grind really starts.