An attempt at this spell that Frogboy suggests has lead me to the following conclusion. He must be laughing because I've killed my sovereign I don't know how many times trying to get a semblance of this spell to work. In strategic mode, just in case anyone is curious, <ApplyToRadiusCenterOnly> seems to work only for certain spells types.
If that can be achieved, you are one of the very few who can do it. Good luck!
The only other things that I can think of to accomplish this is to work it in two different spells. The first spell would kill the champion and give an item to the sovereign, which stores the essence of the champion sacrificed (although I don't know how to store levels or give an item that is level dependent of the champion who died), and then use that item to distribute the XP to all champions on the map.
I'll have to think on this further if I want to try to produce this effect, but at the moment, strategic spell calculations do not reliably work at the moment (bug??) as several core spells use these calculations, but don't actually work because of this calculation problem.
Without being able to use calculations that seems very very difficult...