In Becoming (part 2), what do anda think Whistler means when he says, "Wrong, kid. anda got one lebih thing [to loose]," in response to Buffy stating that she can deal with everything because she has nothing left to loose.

I think he means she still has herself and when Angelus and Buffy are sword fighting Angelus says "That's everything, huh? No weapons, no friends. No hope. Take all that away and what's left?" then Buffy balas "Me"

Isn't it foreshadowing her killing Angel, because he has just gotten his soul back so they would have been able to get back together but instead she must kill him to save the world.

I have been wondering the EXACT same thing for ages now, I've never been able to figure it out. I just figured he meant her life.

I think he meant her "Buffyness", atau her soul, in a way. Later Angel is all, "no weapons, no friends, no hope. What do anda have left?" and Buffy is like "me". I think her losing that spark is also emphasized in the third season opener, when she is no longer living as the slayer.