In my DF Felltower game, there are a number of monsters who can only be (easily) harmed by magical weapons. And some which can only be slain by magic weapons - thanks to Supernatural Durability or Unkillable, usually, with "magic weapons" as the Achilles' Heel.
I've ruled before that the Puissance enchantment is what matters for those purposes - no Puissance, it's not magic enough to bypass such defenses.
Perhaps naturally, my players have commented on the lack of a temporary version of Puissance. Much like Armor is the temporary version of Fortify, and Shield the temporary version of Deflect, why isn't there a temporary version of Puissance? Sharpen is somewhat similar, but it's more akin to a temporary improvement in weapon quality and not the same as Puissance.
I've usually said, yeah, there should be, but there isn't.
I thought about one and realized it is a terrible idea for my game.
Why there isn't a temporary "Puissance" enchantment
There a number of reasons.
First, it would be a must-have spell. No wizard could reasonably avoid getting the spell. What kind of wizard wouldn't take a spell that opens up vulnerability to a broad swath of the most dangerous foes? A foolish or poorly designed one.
Second, it would be a foolish party that delved without at least one main weapon being under the spell's effects. It makes even more sense to have it on every main weapon at all times.You'd want to be ready. You might want some weapons without it, just so you can tell which monsters need it and which don't.
Third, even if the spell was costly to maintain, so keeping it up for free all the time wasn't trivial, that would just mean it was a fight-time casting. And the goal of getting to "free to maintain" would be irresistible. If you decided to make it a costly, short-term spell that can't be maintained and takes time to cast, you're just changing the nature of the effort needed. That effort will still be made.
Fourth, it would mean the immunities and Achilles' Heels were actually over and under priced respectively. It would greatly devalue something meant to be a vulnerability to an expensive ($5000 and up!) enchantment and drive decisions about armament and expenses.
In other words, the existence of a temporary "Puissance" effect would mean there was no real meaning to having monsters slayable only with Puissance. Not just in the game world, but also in game terms. I inserted creatures with that specific challenge on purpose. Adding a spell to undo it in this manner would make that insertion pointless. The spell would make sense if there wasn't that kind of defense, and that defense makes sense given the lack of the spell. Together, they undermine the game about as well as permanent rock-to-gold spells do to a game about treasure acquisition.