The given milestone-requirements for the hardened levels are both a bit random (only because you are midway through Phase x, doesn't really tell you much, which advancements you invested in) but also a nice rule of thumb (because it means you have earned and invested some amount of APs already and have an equivalent set of benefits from them). That said, whether or not you want to start a hardened level before you fulfill the requirement should also include the discussion of the reward you get. Some of those rewards are REALLY useful and therefore worth spending a bit more time on a level rather than reaching the requirement first and then having slightly shorter level times. Which rewards that are, will probably depend on your style of playing. For example, Superpowers can be really great, but if you don't buy powers for coins and have no "external" means of earning powers, the usage is a bit more limited. Others like Seeing Circles are pure convenience. Some rewards are totally useless for a portion of players who don't have/play some aspect (Supercamp, arenas, battles, ads). So, use the forum or this guide:https://docs.google.com/document/u/0/d/1EHCM-n_4vq_Hv7V1wo9QSTcRP2ba4qRqBh8fOh5PiKk/mobilebasic
to see what is hidden behind these rewards and then check for yourself if they are worth it for you.