Armageddon

[Psycho]

I just tested it. It is possible to do multiple castings on the same turn, but it doesn't kill 1/3 of population. It kills 1/5.

So you need 11 castings to get population below 10%.

 pop remaining = (4/5)^no_of_castings * 100%

 1 casting : pop remaining = 80%
 2 castings: pop remaining = 64%
 3 castings: pop remaining = 51%
 4 castings: pop remaining = 41%
 5 castings: pop remaining = 33%
 6 castings: pop remaining = 26%
 7 castings: pop remaining = 21%
 8 castings: pop remaining = 17%
 9 castings: pop remaining = 13%
10 castings: pop remaining = 11%
11 castings: pop remaining =  9%

Still it's hardly worth going over 3-4 castings, especially if you plan utterdark afterwards.