There is an alternative to the wireless mice.

Multibox does allow mouse broadcasting as well, which is not really something I use or want to have on permanently, for the reason that it will cause havoc, but for a blizzard/rain for fire target click it works.

There are two requirements:
1) make sure that you used home/end to swap all chars to the same camera position, which is of course mandatory for the relative location on the screen to correspond.
2) I am not using Multibox's to let my main mouse move to the other screens since that resulted in disconnections for me, so nothing happens when the mouse cursor hits a screen boundary.

Then proceed as follows:
1) switch on mouse broadcasting
2) move the mouse to the one of the screen corners to have them all at the same relative location.
3) trigger the AoE spell and
4) move your target nuke highlight and click
5) switch off mouse broadcast.

I know, not simple, but doable.