but then the 2nd and 3rd mastery missions require 24
Small correction. The second level of the mastery missions requires 25, not 24 mastery levels. And these are in addition to the 12 already done, so you will need a total of 37 to complete the second mission. Given I'm personally only on 6/25, I'm not likely to find out how many mastery levels the 3rd level requires any time soon.
Even to get the second mission done, you will need to raise the average mastery miletone level of all 10 of your pool champs to 3.7 - i.e. you will need to get a bunch of champions to mastery 4+. If there are a couple of champions that you simply have no interest in playing, then you will need to get the average mastery miletone of the 8 remaining champions to 4.6 - i.e. a bunch of champions will need to hit milestone 5+. And that's just for the second mission...
This is taking the ****. I suspect almost no-one will bother to complete the second mission....