I doubt that Demandred's death will be as anti-climatic as you suggest. All of the Forsaken who have died deaths (perhaps with the exception of Asmodean) that were A) significant, and B) vital to the plot. Even Be'lal, who is so often cited as the seemingly least important Forsaken, died a very significant death. With his death came: the fall of the Stone of Tear, Rand's receiving of Callandor, Rand's acquisition of the Aiel. Be'lal's death was therefore necessary and important, though certainly not titanic.

Demandred will have probably a more "titanic" death (explosions, fires, armies, etc) though it will come far too late in the series to truly be monumental.

I do agree, however, that Demandred probably won't die by Rand's hand. As Graendal noted in the prologue, Demandred is single-minded and blind with hatred, which I feel will be his downfall. He will be so focused on the prize (Rand) that he will fail to notice relevant threats.

IMO, at least.