I think that Ishamael's taunting had two purposes. First I am sure that there is a certian part of Ishamael who wants to rub the nose of his great rival LTT that he is still around after all LTT fought for is long gone.

Secondly he was trying to show Rand how outclassed he was so that he would believe he had no chance of victory and would turn. If he could defect Rand's attacks with such ease that he does not even bother to try and hit back very hard that will tell/show Rand that he is not even in Ishamael's league and he better cut a deal with the Shadow while its still on offer.

I believe it was Lanfear who pointed out to the other forsaken that fear and bullying don't work with Rand as he is A)Two Rivers stuborn and B) he has too much of LTT's refusal to be forced into surrender.