Mui and Gui are way different xD , MUI is when two unit can cast the spell in same time whitout any problem and GUI is trigger that you choose action by clicking on choices. Altough , I would like to help you but i'm not on my computer sry.
Since, I don't realy understand what your asking for. I'll make you a map. it will contain two spells , maybe on of those two will be ok.
Currently working on it.
Edit: Want it MUI?
It's normal ^^. the special effect you chose can be destroyed while it appear and it will still stay, secondly, you don't need to "null" variable in GUI so remove "Set ...... = No unit"
your way to do it is bad, it will damage all unit even allies. you need to create a group that will pick every unit in a certain area owned by enemies then you deal damage to picked unit .