Drifter challenge requires you to never buy any upgrades aside from autopromote.
Challenge is pretty simple: you just follow the flow of the game at +00x01 (a.k.a. zombie) and autopromote on each ladder. Throwing vinegar is allowed to improve your positioning and actually is a good idea to use that quite often.
Surprisingly, Winning is actually quite possible but requires a high roll on max_assholes count and a low overall activity.