I remember reading one of the novels set during the time of troubles. Elminster and Khelben teamed up to fight a Demon Prince. Does anyone know who that Demon prince was?
Eltab in Thay is the only demon prince active during the Time of Troubles which immediately comes to my mind. Although I don't recall any direct conflict between him and any of the Chosen, excepting perhaps the Symbul.
Myrkul was slain by Midnight near Waterdeep, so perhaps Khelben (or one of his plots or agents) was somehow tangentially involved.
Accounts of Iyachtu Xvim, the Baneson, are confused. He may have been active in Thay; if so then it's unlikely he interacted with Khelben or Elminster. Or he may have been mysteriously confined, specifically in a subterranean catacomb prison underneath Zhentil Keep (until the destruction of the keep one decade later), again unlikely to have interacted with Khelben or Elminster.
It should be noted that Elminster was rather busy with certain personal problems during the Time of Troubles. I doubt he was in any sort of position to consult/argue with Khelben about the destruction of a demon prince.