Vi∂arr avenges Odin by jumping into Fenrir's mouth and ripping its jaws apart with his bare hands and a gigantic sandal.
Tyr sacrificing his hand to keep Fenrir from destroying the gods, offering to put it in his mouth as insurance that the others binding him with a magical ribbon isn't a trick. It is, and he loses the hand, but Fenrir is trapped until Ragnarok.
Fenrir killing Odin. Not because he killed the biggest, baddest god in Norse Mythology (although that certainly is awesome in its own right), but because after having been bound until the end of the world with a sword in his mouth because of a vague prophecy of his growth causing the gods problems, he finally breaks loose and as vengeance leads the charge against the gods and kills the greatest among them, proving why it was not a good idea to get on his bad side.
More of a Fridge example, but still counts. At Ragnarok, Freyr fights Surtr and dies. However, Freyr is described as engaging him in a "harsh conflict" before he falls. Note that Freyr was completely unarmed and facing Surtr, who was presumably much larger than Freyr and wielded an enormous Flaming Sword powerful enough to burn nearly the entire world. And he put up a fight. The fact that it wasn't a thorough Curbstomp Battle is amazing, but furthermore, some interpretations hold that Surtr dies from the wounds Freyr inflicted on him.
This line from Hávamall: "Cattle die, kinsmen die, we ourselves also die; but the fair fame never dies of him who has earned it".
90 Percent of anything where Thor is involved, such as that time when he ripped of the head of an ox and used it as bait in order to catch Jörmungandr wile being stuck in a small fishing boat. And he nearly succeded killing the thing if his partner hadn't cut off the rope in the last second.