Adding 8 neutrons to 10% of the oxygen atoms in water molecules inside the target's body so the oxygen decays into fluorine with a half-life of 26 seconds, thus turning their blood into hydrofluoric acid, one of the strongest acids known to man.
Basically, inside of a minute, your flesh melts off your bones.
Water elemental magic combined with RL science: Be afraid.
This sort of effect is why one of the rules of magic in my setting is that directly affecting other people is difficult.
Which I am going to what I use for what I am most pleased with—not a specific power, but how power works in general, and I will also use your spell there as a source for examples.
Oh, and as a disclaimer: This is not a critique of how magic works in your world (I don't know enough to even start to do that), you have simply given me a convenient example to use for why that sort of magic doesn't work well in my setting. ?
When ever magic is used against another being, that being's spirit resists. If that being is sapient, it also has a soul, so it is even more resistant. The deeper a magic affects the target, the harder it is to accomplish.
Let's start with the basics.
Shoot an arrow of lightning (or fire, or ice, etc.) at someone? They have a chance to physically dodge, and they get no special ability to resist it due to it being caused by magic.
Those with a strong enough spirit can, however, resist that physical damage.
Try to ignite someone's clothes? Now you have to use your magic to reach inside of their aura and directly affect things in a way that they will instinctively resist. They get to resist both your attempt at magic, and then possibly resist the physical damage.
Attempt to freeze someone's heart? Your magic is being resisted for the entire depth of their aura and spirit, with the resistance growing stronger the deeper you go. If you do manage to get the spell off and freeze their heart, their spirit will attempt to undo that. If they are just some random new recruit, they don't have a chance. But in a battle of a powerful mage and a powerful warrior, the warrior's spirit may keep them alive while their heart thaws, with that thawing quickened by the strength of their will to live and overcome.
In your example, doing that in my setting would require both elemental water and elemental earth magic; while the initial step keeps it as "water, but different", the
intent is to create fluorine which will combine with the calcium in bones to form fluorite. And intent is the single most important thing about magic. It's also a weird enough specific thing that elemental magic is probably the hard way to go about doing it.
Additionally, that sort of conversion would need a more complicated and mana intensive spell than, say, simply turning them into a frog. The difference is that a frog is a known thing and you aren't trying to be precise about the details. You are using the concept of a frog, and because most of the world has some idea of what a frog is, the spell merely grabs that meta-template created by such awareness and attempts to make the target fit that template.
A partially successful spell might turn them into a bipedal frog-man or something, with variability in the duration.
There's no wide-spread template for the concept of adding neutrons to trigger a particular atomic decay to produce a specific (physical) element that will have a specific effect. You are doing all the heavy lifting yourself, and your spell form has to account for everything involved. Which in this case would include all the knowledge of the forces involved and the ability to calculate all the effects using those formulas on the fly. This makes it a very difficult spell to cast mid-combat when you have a lot of distractions, and you can't just ignore those distractions when they represent threats to you.
If you make a mistake, intent can carry you through rather than the spell failing, but it will weaken the spell and make it easier for the target to resist. If you get distracted mid cast, that can weaken the spell, cause it to switch targets, alter the effects of the spell, or any number of other things, depending on what distracted you and what part of your concentration got disrupted.
So, combat spells tend to be far, far simpler than that. The simpler a spell, the easier it is to get the desired effect. It's also easier to, say, change yourself into a giant monster-bear than it is to transform a hostile target into something like a frog, because you are not resisting your own magic.
Side note: buffing transformative spells like that are always built with a time limit. This both caps the mana going into it, and makes sure that you don't have to be able to un-cast it. If you mess this up, the spell could potentially just keep drawing on your mana to maintain itself. And if enemy actions cause your thinking to become muddled, you might not be able to think or focus clearly enough to break that connection. So it's best to just bundle the mana into the casting, and tie the spell off so it can not take up any more.
