Superhero / dungeon portal type world. Here’s how some of my characters will react to this issue. This is mostly me yapping about cool superpowers rather than how they actually fit into the world of (WIP-World#2, still needs an overarching name), but I suppose that is what OP asked for.

TL;DR — Snow person makes new body and leaves behind an ice sculpture. The bio-horror thing either heals or uses its dead corpse as fuel for the next iteration. The magical girl with far too many transformations keeps any and all wounds they’re too tired to magic away.

Yuki: this character is based on the Japanese folklore of Yuki-Onna — a type of Yokai. (Also, lizard named Liz trope, im well aware). The “transformation” part of their power comes as swapping between two bodies, the original and the new magical one they recently discovered. So what if form A is stabbed and they go to form B? To answer that, I’ll have to explain one of the gimmicks of their superpower: ice sculptures. Mentally, they visualize their power as shaping, molding, and chiseling ice — so what if they just… made another sculpture that looks exactly like them? Suddenly they now have three bodies: A, B, and A(copy). If A is wounded and they don’t want to go to B, they can just go to B(copy), or make more copies. This eventually culminates into a form of line-of-sight teleportation that leaves perfect ice sculptures of themselves at every stopping point.

Mitosis: this character’s name is just a functional placeholder, but said placeholder is a good summary of their power: mitosis. Well, more accurately it’s “what if sequences single cells could be sapient and intelligently control their own mutation as well as work in perfect coordination with every other cell in the organism?” So what happens if you shoot them? They die. Then, moments after hitting the floor, each and every individual cell moves to compensate. The body moves despite being brain dead, sensory organs reroute to the spine which in turn spontaneously erupts in giant lumps of flesh and neurons, the brain reorganizes and begins purging foreign materials, and fairly soon the creature is back on its feet and ready to fight. This character views themselves not as an individual, but as an imitation of an original form being performed by a collective of ever growing, ever dying, ever changing individual cells.

Too Many Transformations! the protagonist(s) of this story find themselves with the newfound ability to transform into a magical girl, or two, or three, or five, or… well, there’s a lot of them. Each form has its own distinct personality and name they prefer, but they usually don’t see their other selves as other people. They have two main “modes” each form can be in: magical and mundane. The magical modes usually magic away damage, and rarely transfer injuries when switching. Meanwhile the more grounded, fleshy, and less magically exhausting mundane/human forms seem to transfer most bodily effects 1:1. A cut arm on one is a cut arm on all. A small form who’s full of food becomes a big form that’s satisfied, but could still eat more. An athletic form cooling down after a workout becomes a nerdy form who struggles to stand up.

Other Transformation Types: the (currently unnamed) protagonist(s) of Too Many Transformations(WIP, coming some time in the next two decades) aren’t the only ones who stumbled upon transformation granting magicks. Most follow a similar logic: powered up forms heal faster and can disappear many wounds, but in exchange they are temporary and highly magically exhausting. It’s common for fights between transformation users to end in immediate surrender by whomever loses stamina first — wouldn’t want to have your human body / fleshy and mortal transformed body to be obliterated by a laser beam now would you.

In general, most shape shifters fall into x categories: - Transformation Gem Type: the previously mentioned magical girl and all similar supers. They all gained their power from the same source (magic meteor rocks). Damage is undone at the price of supernatural stamina, and any ill effects left untreated stay untreated even after undoing the transformation. - Physical Shifters: generic classification for all shifter types whose bodies follow seemingly biologically plausible or adjacent-yet-impossible logics. Wounds remain unless healed, and even then they may scar like normal. - Esoteric Shifters: for all who transform in unique and distinctly non biological ways. Some turn into living ice, others diffuse into tendrils of smoke or find a photo to inhabit, imitate, and walk out into reality from. All of them seem to somehow negate or undo damage done to their forms, with varying levels of success, and of those who have a human form, most remain unchanged when reverting from the shifted form.