Merela strangled one of them to death with her bare hands. Primordials don't need to breathe.
Thousand Faceted Nelumbo wants to master Sidereal martial arts. She knows this is supposed to be impossible, canon says it's impossible and the default assumption is that she will fail. On the other hand, Exalted are known for doing the impossible.
Two notable subversions: All Abyssal Exhalted know they can be redeemed, just not how (G Ms are encouraged to make it up and make it hard). Also, it is impossible to reach Malfeas (Hell...ish) in less than 3 days. There is a charm allowing a Solar to punch somebody straight through the sky and immediately into Malfeas...where the person lies in stasis for the three days it takes to get to Malfeas.
This is how Solars Exalt. A mortal realizes a task is impossible, then does it anyway. Other Exalted have elements of this in their own Exaltations (Lunars have to reach the Despair Event Horizon and keep going anyway), but Solars are the most explicit.
The mass of Tyranids in the hive fleet is so incredibly huge, the subconscious buzz of all their minds adds up to drown out the Warp. This renders all FTL travel and communications impossible, and presumably also makes Daemons shit themselves.
The Necrons are an entire race of (mostly) humanoid automatons made of regenerative metal who carry theoretically impossible molecular flaying energy weapons.
Due to missing out on the mass Mind Rape inflicted by the Nightbringer on the galaxy at large, the Orks alone are a sentient race entirely free from the fear of their own mortality. They do not fear death at all, and indeed rush heedlessly towards it and love every second, but they still fear Commissar Yarrick!
The 6th edition Chaos Space Marine codex mentions that Warp Talons have blades so sharp that they can slice the very fabric of reality itself.
In the RPG adaptation, Dark Heresy, permanently expending a Fate Point will allow your character to survive anything — any one attack or situation that could prove fatal, doesn't (although just barely). For instance, a character may spend a point to downgrade a lethal lascannon shot to "merely" dealing tons of damage and knocking them out. The rulebook mentions that this can apply to any situation where one might die, no matter how outlandish, and conversationally suggests that it might require a bit of odd justification to avoid dying on a ship that explodes while in Warp transit.
Planescape features razorvine, growing mostly on Lower Planes, but eagerly acclimatizing anywhere. Its stem is a living razor wire, growing twisted and under tension and thus lashing around if cut. Some try to use it as a security measure, but it's a virulent weed and hard to eradicate. In some places, any sod barmy enough to bring in any plant cuttings can be summarily executed. Egarus is a fungus from Abyss which was accidentally introduced on a Prime world, and after discovering that it grows everywhere and they can't even Kill It with Fire. Natives managed to kick it out to the Quasiplane of Vacuum... and it survives there.
Two of the cards that can be played to make combat difficult are Last of Its Race and And Its Clone. They can both be played on the same monster. note There's also Mate and Brood, both of which would be rather illogical to play on a monster that had been made Last Of Its Race. Munchkin Cthulhu also adds "...and its Spawn." Or Last Of Its Race and Is Your Father... Or just Is Your Father, period, given some of the monsters you can throw it on. Like a comfy chair. Or the aforementioned 3,872 Orcs. All of them are your father.
"Cheat With Both Hands" (the seventh expansion for the main game) is this trope in card game form. Along with the titular card (which lets you cheat for two items), there are cards which provide extra or even unlimited Races and Classes.
The card called "Cheat" allows you to disregard all restrictions when attached to an item card. Depending on how loosely your group interprets the rules, this may lead to physically impossible things (such as wielding more weapons than you have hands).
Warcraft: One of the novels reveals that after the War of the Ancients, Deathwing tried mating with his consorts; his deformed, fiery body was so hot he burned three of them to death and horrifically scarred the fourth. For reference, black dragons are immune to fire and regularly take baths in molten lava.