ISBoxer has an example key for this in FTL DPS Keys, "Example targeted AOE". It does the following:
1. Keystroke (to activate the spell) -> all
2. Send Next Click -> all other
.. so you just hit your hotkey, it activates the spell, you aim and click with the mouse to finish the job.


If only a handful of your guys have this AoE (or equivalent) to cast, you can set up an Action Target Group. Put your guys in it and send the keystroke to Group (all of):My AOEer ATG, and the Send Next Click to Group (others in):My AOEer ATG.


To make aiming in all windows easier, either what Khatovar said or personally I just manually point the camera angle on my slaves straight downward long before casting. This also has performance benefits since the slaves don't need to render as much.