Add me to the list of people who likes the idea of two varied effects. For instance a death spell could do so much hp damage on a failed save and if the monster's remaining HP are above a threshold it is weakened (or the equivalent) until the end of its next turn, if its remaining are below the threshold then it dies. It's useful for the BBEG & can potentially kill him if used right, but it will still instakill a standard monster which is completely appropriate.
In my opinion, if you're going to have a HP mechanic to determine how close a monster is to being defeated why not use it? Let the party whittle down the BBEG so it can be disintegrated by the wizard.
Another option I have played with in past games as a house rule is to severely adjust saves so that it is much easier to save vs. a spell, but a spell that doesn't work isn't lost. For instance, if it takes the fighter with a 75% hit chance three hits to kill a monster (assume four rounds to death), make it so the monster only has a 25% chance of failing its save vs. that petrification spell (again four rounds to death). But this was pretty hard to do in previous editions, it might be even harder to do with bounded accuracy and varying classes of monsters likes standards and elites.
I do, however, love the idea of an at-will petrification, polymorph, or other instant kill spell that the wizard just spams doing no damage over 2-4 rounds until it works though. It makes it feel like the fighter is engaged in a deadly battle of blades while the wizard stands there engaged in a prolonged battle to impose his arcane will upon his opponent. It works at the Vancian level too, just make a spell sustainable with an action on subsequent turns.
I like the HP threshold because it is faster, but the reduced save chance has less metagame issues and really reinforces the idea of a caster being about concentrating until they have imposed their will. Its less effective vs. mooks though, it might be worth adjusting saves based on the level difference between the caster and the target.
In my opinion, if you're going to have a HP mechanic to determine how close a monster is to being defeated why not use it? Let the party whittle down the BBEG so it can be disintegrated by the wizard.
Another option I have played with in past games as a house rule is to severely adjust saves so that it is much easier to save vs. a spell, but a spell that doesn't work isn't lost. For instance, if it takes the fighter with a 75% hit chance three hits to kill a monster (assume four rounds to death), make it so the monster only has a 25% chance of failing its save vs. that petrification spell (again four rounds to death). But this was pretty hard to do in previous editions, it might be even harder to do with bounded accuracy and varying classes of monsters likes standards and elites.
I do, however, love the idea of an at-will petrification, polymorph, or other instant kill spell that the wizard just spams doing no damage over 2-4 rounds until it works though. It makes it feel like the fighter is engaged in a deadly battle of blades while the wizard stands there engaged in a prolonged battle to impose his arcane will upon his opponent. It works at the Vancian level too, just make a spell sustainable with an action on subsequent turns.
I like the HP threshold because it is faster, but the reduced save chance has less metagame issues and really reinforces the idea of a caster being about concentrating until they have imposed their will. Its less effective vs. mooks though, it might be worth adjusting saves based on the level difference between the caster and the target.
