Flamestrike
Legend
That's exactly what I meant by "it's another thing for the DM to just fiat the Balor into existence in melee range." I get it, you love to use your DM's Fiat to provide hack-and-slash fodder so that your players have lots of casual murder several times a day to give them "deep character development."
I'm not going to tell you not to, but I will point out exactly what it is.
If an enemy wizard really did it to get revenge on the party, he'd do it more effectively by catching the party wizard at a bad time when the wizard is alone and going to the bathroom. No, you're just handwaving because you don't care why things happen as long as the party gets to make attack rolls. It's not necessarily a bad way to play, but it's not a universally-desired play experience either.
I don't want to ruin the surprise for you, but every single encounter is DM fiat.
You the DM decide what monsters are going to be there, why they are there, the number of monsters, the composition of the encounter, its difficulty and challenge rating, where they are, encounter conditions including encounter distance, the disposition of those monsters etc.
It's no more DM fiat to have a Balor plane shift in to trigger and commence the encounter than it is just having it sitting in a room all on its own twiddling thumbs waiting for the PCs to show up. Heck the plane shifting balor is probably a better encounter. Smart PCs will start thinking why was it after us and why did it target us? It also keeps them on their toes and keeps them guessing.
I can't give you a mathematical solution for how to be more creative or to be a better Dungeon Master. It's an art is much is it is a science.