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