in the level 'Sibbling Rivalry' carnage can't be defeated when he is nocked down. Does anybody now how to defeat him?

You have to throw the gas cans when he's at his lowest health.
Hope it helped.

back him into wall using combo ( x works best).
When Carnage is knocked down when he gets back up he makes a noise (like a rubber glove pulled back), as soon as this is heard launch the x combo until his life is very low.

When this is done Carnage stands still so use R to consume him which gives you back some life thus defeating Carnage.