See topic.

I was thinking of large monsters like:

Medusa - capable of Stone Gaze (instant death) - weapons crafted from her would have the tiny possibility of an instant kill.

Kraken - extremely powerful leviathan-type dragon. Not as impervious as from Greek myth (Perseus killed it with Medusa's head.)

Scylla - from Homer's Odyssey - a six-headed sea monster.

The Nemean Lion - Heracles' first labor. His hide is impervious to all blade attacks (fight him with a blunt weapon like a hammer or switch ax.)

Trolls or Giants - from Norse mythology