I have written this as a cautionary word of advice to one player in particular, and as a guidline to players in general. Feel free to comment back to me and other on any thoughts this might provoke:
Life is not cheap. Indeed, immortal life is a dire thing to lose. A duel rarely ends in death, and assasination is an extreme.
The goal of an ambitious Amberite or Chaosian (I will hereby say Chaosian to mean both) should not be the death of their enemies. It is easy to kill someone else, and it is, frankly, boring. If everyone in the Courts simply killed their enemies, few would be left, and after a century or so, they would keel over themselves having no one else to play with.
Shadow people are utterly meaningless. They are nothing. But even an enemy Chaosian should be treated with respect when it comes to their life. To kill your opponent is not to beat them. They could have as easily killed you, by subterfuge, hire, or magic. To beat your opponent you must humiliate them, you must make them give something to you. This could be anything from an apology to something of value they own (it would have to be of personal value, since everything of monetary value is infinite in shadow). You must beat them by heart, head, or hand. You must outwit them, outsmart them, and outshine them. You must compromise them in front of their lover, force them to lose honor in front of their House, or to back down in front of their brother. You must make them lose power, prestige, or confidence. And you yourself must remain impregnable all the while.
These are the victories to be had. Beating a shadow dweller at anything is nothing, and should only be viewed as training to beat a real person, an equal, a Chaosian. Take a century to hatch your plan, and still make it as subtle as forcing them to lose a pawn in a chess game, for that is what the battles of the Courts are made of. Amber is even more subtle than the Courts.
A duel is ugly, and should only be employed when personal honor and strong feelings are involved. Besides, they aren't very strategic. One of two people is going to lose each duel. If you duel someone inferior to you, you gain nothing in others' eyes when you win. If they are superior to you, you will lose. If they are your equal, you only stand half a chance of winning, and you should devise a plan with a more likely chance of success.
But not killing, only in the most extreme. Killing is actually considered wrong in the underwritten laws of the Courts. In war, fine. In personal conflict, there are so many classier ways of dealing with foes even worse for them than death.
Destruction is the same. Dworkin lost favor when he destroyed the balance that existed before Amber. When Brand told Corwin that he almost killed him once, and only his favorite rug which Corwin was standing on prevented him, Corwin did nothing. But when Brand threatened to destroy the universe, everyone got off their asses and put the man down.
It is bad form to threaten the universe. Poor ettiquette. Almost an immortal faux pas. Kill if you must, destroy if compelled, but recall that it isn't clever, difficult, or admirable.
But making someone apologize in public, that will be remembered, both by those who witness it and by he who you forced to do it, for decades to come.