You will unlock this after you beat the final boss, who you will face immediately after beating the fourth of the other four bosses.
Boss battle tips:
You will go head to head with another car. Put most of your focus towards making good turns and not crashing or running into the wall. Use your boost if you find yourself behind. Using the boost while you are in the lead is somewhat pointless because the CPU cars always manage to catch up to you again, so save it. If possible, try to save some boost for the end of the race to give you a little push, making sure you win.