Ask a Thief Thread.

I do that at LHC with low levels, but when I train with high levels, they all kill it before I can even explode so I turn PP off. So my only options are linking BS+PS or BS+Steal, and steal is

That's why Stronghold is so much better. There's literally always something to kill.

I don't think you should be using assassinate for training (not 100% sure though, since I'm assuming you'll max assassinate pretty late and maybe be in stronghold where you'll never need to use single target skills by then) and you don't need speed for bosses really.
As for how I like my thieves, I find wind charm quite useful on my NL, somewhat so-so about replacing avenger with shade split, annoyed by shadow star's having an actual delay, love my shad's DPS (near cap with assassinate using 5 orbs without darksight so I can keep my haste) and lastly I haven't really gotten around to my DB yet, but it didn't really change much yet.

While mobility in bosses is nice (HT and whatnot), it's not as big of a deal. My main complaints are with Dojo, where mobility matters and can knock some time off.