With his "special gifts" from the DO, being an experienced Top Assassin for the Shadow using deadly poison & projectile weapons, who had a Warder training as Luc and probably badass Malkieri "royal genes" as Isam (who apparently even easily survived in the especially deadly high passes of the Mountains of Dhoom, which even the super deadly Worms fear, according to the Dark Prophecy).

Besides that, the usual argument is that Asmo could easily stop any non-channeler with the OP, before he could hurt him. That is obviously wrong, according to RJ. Certainly Slayer can act at least as fast as Fain and easier manage to completely surprise Asmo by suddenly appearing out of thin air.

Therefore, I definitely think one can compare Fain and Slayer in terms of deadliness and the ability to kill Asmo in this situation.