I think I might go for the Telling Blow feat, and try to fit in Improved Critical as well (Imp Crit and Keen stack in my game), and give the Tanarukk a +1 Keen Scimitar. With a +1 weapon he gets 3 attacks at +18/+13/+8, so that first one will likely hit, and if he rolls a 12 or better on the die (and confirms), then he gets sneak attack.
The other thing I might do, is give him a scroll of Rope Trick. When the PC's deal with the mooks downstairs, the assassin may well hear them. He could then hide in the rope trick and deal sneak attacks at range from in there. My PC's are likely to notice the window into the rope trick (they like running around with Arcane Sight up), and may well dispel it (actually, I don't think you can dispel it so long as the rope is brought up with you), but he can just as easily cast Vital Strike, etc, up there and tag the PC's as they wait below.
Hmmm... Suppose the assassin did "hole up" in the rope trick. What can the PC's do to get him out?