The key of beating all 12 is not taking ANY real damage in the first 9(which in turn, sweep helps, so it goes in full circle), so that at the last 3, if worst come to worst, you can afford to throw an entire team to just soften up the enemy (kill 1-2 char)
To accomplish the first part, you need to have a team with good sustain(heal/shield) and damage mitigation(interrupt enemy mystery, hinata, kimimaro etc).

You should pay attention to enemy ninja and formation and adjust your formation to counter them accordingly every fight(dont hit useless clones unless there is a reason to, also avoid earth main or garra if you are trying to proc a chain off normal attack etc). If you are having trouble, you c*e repeated retries to get a lucky proc off a unit to start a normal attack chase chain(and prevent enemy from doing so).

I'm not sure on level requirement for sweep or health requirement. I do know, however, to get the full sweep you need to clear 12 repeatedly. After first one or two, you get 3, then after another full clear or two, you get 6 and finally 9 after yet another full clear. Fail once or skip a day (or even reset and not clear in a day) will get you reset to nilch.

For one, it will only be there the day after completing 12, so if you completed 12 from a previous day's reset and THEN reset, it wont be there.
For another, I'm not sure if it will be there right away after just one clear, it might require more clears to unlock (and at that, 3 at a time, not all together)