Thug, binder or poisoner. Trapper and Pickpocket are largely defensive. Poisoner is considerably more of a pain in the ass to get right, as the thief.

Pre-trance you have a strong advantage against assassins, unless you're outgunned in terms of gear and skills. Especially assassins with a vuln. As the thief, you're probably human.

If you start with a knock out attempt and fail, you just run away. Most times you'll want to start with a knockout attempt unless the assassin really, really sucks, which case you might either backstab or just trip spam him to death.

That's what people are missing about the matchup. Assassin can't really keep thief around. So thief takes his shots. Most of them probably don't work. Some do. Ideally he does this with very little risk to himself.

Post-trance, for the most part you'll need to get the drop on the assassin and knock him out before he sees you and trances. Easier said than done, depending on how paranoid the assassin is.

Pick some top-tier thieves and load up their PBFs. Check out their records vs. assassins. I just looked up a few and the tend to have better PK% against assassins than they do overall.