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

he means the last thing she has to lose is her life (:

I think he was talking about ángel .. Her having to kill him

I think it has two meanings:
1. That she still has her own life to lose.
2. That she'll have to give up on any chance of getting ángel back, o Whistler had foreseen she'd have to send ángel to Hell while he was ensouled.