All that mana is being drained from the people?
If that is the case, then they are already dead.
At least most of them should be.
It either drains it evenly, in which case the people with lower mana pools are already drained
Or it drains it completely from 1 to 1 to 1 individuals, in which case they are also dead.
These spells are very taxing on even the monsterly amount of mana that MC even has access to, there is no way normal people can support them more than once or twice.