On the flip side, when I was an assassin trying to kill thieves, I would a. be a felar, b. wield a staff, and c. pick edges conducive to killing thieves. It didn't work very often, because (as noted above) assassins can't reliably perma-lag.

I see two approaches, as the assassin. If the thief is someone you don't think will stick around if he misses the knockout (or if you surprise him) then you lead with a one-round skill (or maybe trip) and then spam throw and hope he kills himself by spamming commands. If you think he'll stick around for a fight then you can try the "owaza surprise" method. Given thieves' low hp and the lead time on owaza you could potentially make it your first command after combat starts.

Just looked at my hero assassins. One had an infinite kill/death ratio (some kills but no deaths), and the other's ratio was between 1 and 2, which was lower than that character's overall kill/death ratio against all opponents. Of the two of them, the "worst" kill/death ratios were against anti-paladins, invokers and necromancers. I didn't take the time to isolate hero-range only; it's possible the ratios would look different if only hero-range fights were considered.