how does the description of Sir lucan's death contrast with the speech in which Arthur bemoans his passing?

I think irony might be a better word to use than contrast. Lucan tries to help Arthur but is actually in more need of help himself. When Arthur finally comes to (after being helped by Lucan) he witnesses Lucan, "foaming at the mouth and part of his guts lay at his feet." That's pretty grim stuff. Arthur sees Lucan as noble in his speech so rather than contrast, I see irony.