to kill carnax easy,u need to be a guardian and a lvl 9 necromancer.
you should have the ability undead mutant.
when ur about to battle carnax there would be a treasure chest next to the carnax battle screen,open it and u will get a temporary carnax blade this does chaos damage to carnax.
next battle carnax and use undead mutant on him it would most likely do 60-150 damage and because of undead mutant u will do 6 hits.
it would be realy easy to beat him,plus u get easy xp and cash,how about that >:P